Benefit of a Newer (Used) DAC - "Bang for the Buck?"

Unfortunately, it is like asking us if you will like Mutsu or Fuji apples better. I have no idea and no one else will either.

If you are looking for near the most accurate and uncolored sound possible, your current CD player into a Qutest or Benchmark DAC would be a significant step up from where you are now. Heck running the Time Lens with the dither off into a $150 Topping E30 would be a big step up in performance from what you have now.

If you love the sound of vinyl no matter the genre or performance, then there are probably better options out there that will more closely replicate that vinyl sound.

If you are looking for where to spend money, it is almost always acoustics.  Another area in your case, though, is the crossovers in those speakers. Belle's are from the early 70's. If those crossovers have not been updated / upgraded, then they are well past their time. The electrolytic capacitors specs would have changed considerably over that time to the point where the crossover points could have moved quite a bit.

Maybe read up on the RME ADI-2 . I've had demo model for a week. It's real nice. 

Great very transparent PEQ as well. 

Actually, @audio2design, you've answered my question:  Is money expended on replacing an old DAC likely to result in an audible upgrade?  Sounds like it will - even it the replacement is an entry-level version. That's all I needed to know; I can now move forward re: model specifics.

For the record, the Belles I own were manufactured in 1984 - and have new Crites AB-2 crossovers.  The original Klipsch AB crossovers in these speakers were not particularly good; Bob won't even rebuild them. Production of the Belles was phased out in ~2005.

There are a lot of excellent DACs below $1K (new and used). In addition to the ones mentioned, I'd look at the Denafrips Aries II (new for about $800) and the Schiit Gungnir MB (used for about $950).