CD Player with the best Built-in Volume Control...

I'm looking for a CD Player with an excellent Built-in Volume Control, to use via direct-connect into Amplifier (Pass Labs X250.5 > Wilson Audio Sophia Speakers). Any suggestions?
Thanks for giving us information on the rest of the components. I'd strongly recommend a Theta Miles, balanced version. One of my current systems uses this to drive Portal Paladins. A similar system driving a Pass amp has been very satisfactory for a buddy of mine. Used Miles (balanced) tend to go for $800 to $1000 on Audiogon. Because the input impedance of Pass amps is on the low side, I'd suggest you avoid players (or preamps) whose output impedance over 300 ohms. 100 ohms or lower is better in this case; although, at this moment, I can't tell you the output impedance of the Miles ...
Cayin units have remote volumne.
Meridian 808.2
APL Denon 3910 or APL NWO3.0
The Raysonic CD-168.
Four others: Linn Genki, EAR Acute, and Ancient Audio Lektor Prime or Lektor V.
Audio aero Capitole is a very musical player with a built in valve pre. Not the greatest build quality but sounds great. It upsamples and has inputs both analogue and digital. Its fully balanced too.
Resolution Audio. The CD-50 is the model I've used but I expect the later models are even better.
Well, the two that I have knowledge of are the Audio Aero Capitole Mk II (used price around $3.5K) and the Resolution Audio Opus 21 (used price around $2.3K). Both of these cd players have an analog sound, which appealed to me, as I prefer much more analog to digital. (As opposed to some cd players, such as the Ayre C-5xe, which sounds more resolving, but also more digital sounding. I guess there are always tradeoffs, huh?!)

The AA Capitole is a great cd player. It is slightly better than the Opus 21, (as heard in my own system), but since used it runs half again as much as the Opus 21, it should be. The Capitole II is fully balanced, as is the Opus 21. Note: the Capitole II is top loading cdp, (and requires the use of a "puck"), which I'll be honest and state I did not like nearly as much as the front loading Opus 21. The Opus 21 does allow you to bypass the volume control, (for the very best sound), however that requires the use of the DIN output, which then requires a special interconnect. (I own and use the Opus 21 using the DIN output, with a AU24 interconnect.)

IMHO, it was not worth the extra money in order to gain such a slight improvement in sonics. But then again, I am mostly into analog, so the Opus 21 is "good enough" for me.

My recommendation is if you have the money and digital is your primary source, go for the A.A. Capitole II. If you're looking for a very good digital source, to pair with your analog source, (and wish to save some money), go with the R.A. Opus 21.

My two cents worth.
Audio Aero Capitole II

It can also serve as a pre for DVD, Satellite, and (latest version)--has analog inputs for a tuner.

>>Just not a pre for vinyl<<<

Great volumn control with tubes .
I would say, that it is silly to select a CD player, just
because of it's volume control. I have added high quality
volume controls, to several different CD players, and power
amps. I highly recommend the "ladder" style.
I don't know what your budget is, but I use a dCS P8i into Watt/Puppy 7s.
Raysonic 168 variable tube out is soooo good I eliminated my $4K preamp
I think you would find the DCS p8i mechanical if compared to the Capitole M2 se.

I have used both. While the P8i SACD player has amazing detail and leading edge information, the sweetness and smoothness of the AA Capitole will seduce you. If you like the more analogue sound, the AA capitole is the one to go for.

To go the DCS route you need to go further up the line. They make amazing players, but there must be no compromising in your system to to get the best out of them.
Better late than never - for those doing research for current day purchases:

I've owned many of the digital systems mentioned within the various responses. Additionally, I have owned Pass Labs amps (Alephs + a 150.5) and have owned Wilson speakers (Cub, W/P 3/2, W/P 5.1, Duettes).

DCS into the amp and into the Wilson speakers is very, very good. I have to assume that since you own Wilson speakers you want a detailed, forward sound (even though the Sofia are a bit less forward than the W/P).

The AA Capitole and will seduce you, it is beautiful, full, bloomy, bloated in the bass by comparison, very, very warm sounding and lacks some detail as a result. This is typically a CDP that is contrary to what many Wilson owners are looking for. It is a great additional CDP, but this warmth may wear/bore on you over time. Also, you may want to buy two for when it will inevitably be in the shop for repairs - in France were they work very sloooooowly. I have one in the closet that needs to be serviced for the second time now. Replaced the entire Stars system and tubes the first time.

The RA 21 is only special with the GNSC mods, then it is quite good for the used price, but in a different league vs. the DCS, EMM Labs or AA units.

If you want detail then I would add Esoteric and Wadia for the forward sounding wants.

For the neutral minded, I would include EMM and DCS and possibly (but at a noticeable step down) the RA 21 GNSC'd.

For warmth I would include the AA (probably two units to be safe) or better yet, the AMR units. I would tend to skip the BAT or Cary in this category as vs. the AA and AMR you get a more stereotypical tube warmth with a much more notable loss of definition.

This is all just in my opinion. I have owned every brand/model referenced except the BAT and Cary which I have auditioned several times.