Hi Steve,
First of all congratulations to your BC9500, it is a B&O classic and my favourite system. I have one as my main music system and I don't want to change it.
I don't want to sound too harsh, but in my opinion the person who told you that Beolab 8000 outmatches Beolab 6000 only in a large room and at higher volumes did:
a) wanted to sell you a pair of BL6000 instead of BL8000, or
b) is more than partially deaf on at least one ear
Beolab 8000 has definitely more bass and a fuller sound picture than Beolab 6000 and the difference is notable even at low volumes. Active speakers (like the Beolab range) do, in my opinion, not really need high volumes to show their abilities. I suggest you do a bit of research in the forum about BL6000 vs. BL8000 (the difference has been discussed many times) to get more opinions on this.
My main living room where the BC9500 is setup is rather small (about 20m²) and I did a bit of comparison between a pair of Beolab6000 (that I bought two years ago for the office, but then I changed plans) an a pair of Beolab Penta (my main speakers) in the same room/position both connected to my BC9500. BL6000 has decent treeble and middles but does definitely lack performance in the lower frequencies.
I won't go deeper into the (really big) difference in performance between BL6000 and Beolab Penta as a direct comparison would be a bit unfair (Beolab Penta is situated above Beolab 8000 in the B&O hierarchy) but listening to the same piece on both speakers does show the weak spot of Beolab6000 (low frequencies).
Don't get me wrong, Beolab6000 for itself is a decent speaker, but if you compare it to it's bigger brothers like Beolab8000 or Beolab Penta (I won't mention Beolab 5, Beolab 50 or Beolab90 as they are a completely different league), you will notice the difference in the lower end. Paired to a small subwoofer like Beolab11 does really help increase the performance of Beolab6000. Between Beolab6000 and Beolab8000 I would always choose Beolab8000.
When you plan to buy a pair of BL8000 just be aware that older pairs tend to have problems related to the deteriorating damping material (like Beolab4000), but this can be repaired.
Greetings from Vienna and happy new year!
Regards
Foxtrott