Hi Andrew,
Yes, I suspect your converter as well. The problem is that downscalers are a very mixed lot, and that price is not necessarily a good inicator of performance (i have seen the same box re-branded by atlona for x3 price).
I would look for anyplace you can 'try before you buy' ; maybe amazon where you can return a product as dysfunctional. Or maybe you can borrow a downscaler ...
As I said, I would go with a HDMI based solution as gear is easier to find. If you MacMini is recent enough you can even send audio through the MDP->HDMI cable. A best case scenario would involve a downscaler with HDMI passthrough as it would save you a box.
Passthrough HDMI would go DVI+Audio and Downscaler would go to SCART. I had something very similar running with BY7-40 mkV a few years ago.
Good luck !
- Scott