Transport and DAC or Upgrade CDP?

In my opinion, Jolida 100A is not an upgrade from what you have -- it's just a way to add the coloration of tubes (which you might like, but it's not by definition an improvement). My suggestion -- keep the Music Hall player and use it as a transport to feed a Benchmark DAC1. If you buy used, try to get the latest model of the Benchmark that you can find (manufactured within the last year or so if possible). You can buy direct from Benchmark with a 30-day return period. I own two Benchmark DACs. I prefer the sound of the Benchmark -- clean, neutral, detailed but not irritating -- to some fine players I've owned, including Cambridge 840 (a superb player, which you might prefer to the Benchmark), Rega Apollo (nice sound but not to my taste ergonomically), and Naim Nait CD5i. If you're looking for a warm and romantic sound, then the Benchmark might not be to your taste, but I've found it to be a really fine device, true to the music.

You asked, " how do I evaluate my two CDPs as transports?" Either will do the basic job, especially if you get a DAC with excellent jitter rejection (like the Benchmark that I recommended in an earlier post to this thread). I once owned a Cambridge 640D and found that its transport was mechanically noisy, making high-pitched whiny sounds audible during quiet passages of music. That might have been the case only with the unit I owned, which sat only a few feet from me in an office system. I've read that the Music Hall transport operates smoothly and quietly, but I haven't tried it myself. You have both units on hand, so you should keep the one that works best for you. I'd probably keep the Music Hall since, as you noted, it sounds better than the 540D, and who knows, after trying DACs, you might just decide that you're satisfied with the Music Hall on its own. At least you'll know whether the DAC represents a significant upgrade to your ears (which is what matters). By the way, you'll need a 75-ohm digital cable -- I'd suggest you go with one from Blue Jeans Cable (Google it) or from Canare (advertised on Audiogon, search "Canare") -- both are excellent performers, good values, and soundly engineered, free of snake oil.