Free speakers?  Therefore cheap new "sub" woofers: Definitive Technology ProSub 800, $250 on amazon.com.  Maybe Jamo S 808 Sub, $159(!) on amazon.  8-inchers cannot play extremely loud (without distorting) or truly low, but that's perfectly appropriate for mating w/ BL4's.  Both come in white and both have fabric grille covers -- important for unobtrusiveness.  There are many other choices, for example with smoother output but more expensive (e.g. REL), or louder & deeper but more ugly (Monoprice, AudioEngine).  If you have a good hiding place, the cheap Yamaha YST-FSW ones fit under some furniture.  None of these will shake the room, not even 12x16', not for your electronica.  But if they are invisible, they will give you quite a start if all you see is the BL4's, when Dennis de Laat's "Sound of Violence" kicks in...  

[Edit: Caveat: I have actually used the Yamaha and DefTech long-term.  Not the Jamo -- just saw it scanning around and thought "That's cheap even for an 8", and it doesn't look horrible like most!"  The Yamaha is under my desk, used at low volumes for near-field w/computer speakers.  A pair of the DefTech's are offloading the bottom octaves from KEF LS50's.  If you crank them on hardwood floors they will jump around!  But on carpet and for musical purposes rather than movie sound effects, they are fine.  I have heard, but never owned, RELs (various sizes) and AudioEngine.  I have never heard the Monoprice but they have a "cheap but acceptable" reputation, and get OK reviews.]

Hi J,

Welcome to Beoworld!  The good news is that you can use these great little speakers with any brand of active (ie. powered) subwoofer, it doesn't have to be B&O.  The best option is to look for a sub that has red and white RCA Audio Out sockets fitted, then you can connect a pair of Beolab 4 or Beolab 4PC speakers to these sockets with this cable:-

None of the models I mentioned have RCA outputs.  Most take a separate subwoofer "LFE" input, requiring the receiver to do the crossover.  The DefTech also has *speaker-level* inputs & outputs.  Of the inexpensive Monoprice units, only the 12in. 150-Watt has line level outputs, alas it outputs a *full-range* line-level signal, to daisy-chain *subwoofers*, not (so far as I know) to split off at the crossover frequency.  Anybody?

Hi Steve,

Thanks a lot for your guidance, this is really helpful.

I live in Berlin and have been looking at what is available on the local version of eBay. There are literally hundreds of subwoofers. I understand what you mean by a sub that has red and white RCA Audio Out sockets fitted. However, it's hard to know exactly what the other qualities of the sub should be. Your cable product linked says 'Suitable for variable level "pre-out" RCA sockets only.' 

I can find subs which have output RCA sockets, and also ones that have output sockets that are not RCA but seem to require the wire to be inserted and then screwed shut under a knob (not sure the name of this type of socket). Some subs have both. In some cases, the RCA output specifically says 'low level output' and the screw shut ones say 'high level output'. I assume I want to be sending the high level output to the BL4s, but this means it would be coming from a screw shut knob. 

Some other subs are more basic and don't specify so much.

So far I've identified the following, all seem viable. The top one seems the best based on price new and the number of dials on the back of it - in case I need to start adjusting phase and crossover etc. Canton seemed a good brand and it helped narrow the selection a bit.

https://www.ebay-kleinanzeigen.de/s-anzeige/canton-subwoofer-as-85-2/1211317504-172-3404

https://www.ebay-kleinanzeigen.de/s-anzeige/canton-as-200-sc-subwoofer/1209020947-172-3412

https://www.ebay-kleinanzeigen.de/s-anzeige/canton-subwoofer-asd-220-sc-kaum-genutzt/1213024824-172-3480

https://www.ebay-kleinanzeigen.de/s-anzeige/canton-subwoofer-asd-250-sc/1219205257-172-9652

I'd really appreciate your guidance on more specifically what type of sockets I require on the back of the sub. I am willing to pay up to about 200 euros, but I definitely want to get a second hand sub. 

I am also definitely up for using Sounds Heavenly to ensure I get the right cables!

I'm pretty technical and enjoy getting involved with detail on this.

Thanks,

Joseph

Hi Joseph,

Thanks for your reply.  Any powered subwoofer with red and white RCA output sockets should usually be suitable for connecting to Beolab speakers, as these will have their volume controlled by the music source or TV.

Kind regards, Steve.

josephacthomas wrote:

I can find subs which have output RCA sockets, and also ones that have output sockets that are not RCA but seem to require the wire to be inserted and then screwed shut under a knob (not sure the name of this type of socket). Some subs have both. In some cases, the RCA output specifically says 'low level output' and the screw shut ones say 'high level output'. I assume I want to be sending the high level output to the BL4s, but this means it would be coming from a screw shut knob.

DMacri wrote:

If we look at your first subwoofer selection, the sub has line level and speaker level ins and outs. It also has selections for crossover frequency and phase adjustments. Even the auto on has selections for which level inputs you are using.

The crossover adjustment sets the frequency the subwoofer will hand off the signal to the Beolabs.

Hi,

Thank you to all repliers on this thread - extremely helpful advice.

Canton:

In response to this specific information: further research by me into Canton suggests it might not be the best brand to be looking at because a) 'Low Level Output' as you have stated in fact gives a full-range signal and b) "Low Level Output Filtered" is a socket option that I cannot find on any Canton model available online.

I will actually get in touch with Canton to see if their 'Low Level Output' does in fact mean, in some cases, that the output is filtered. If it is, then it opens up the options to buy a Canton.

ELAC:

The only other option I have found is this one:
'ELAC SUB 101 ESP'
link

And going from a 1-page manual I found (), this ELAC does indeed filter line level out. It says, in this document, 'Outputs Line: active filtered, 1st order; LS: passive LF filtered with 220µF'

Well done!  This may seem cold, especially spending someone else's money, but... at 99 Euros for the ELAC sub: buy it and try it!  Even if it has a fixed line-out crossover frequency, it's still better than the other options you have considered.  (My personal opinion: why say "active" and list the filter, unless it is variable.  But I know nothing...)  ELAC does run their own forum on their website, which is where I would ask...  Then please publish the answer here!