Tuner + CD --> X-Pre-->X-Cans isn't the best idea. The X-Cans is essentialy an integrated amp,...you'd have the volume pot for both the X-Pre and the X-Can in the chain. Many/most preamps have an output that's not affected by the pre's volume (a tape output for example), but the minimalist X-Pre does not sport such luxuries.
The Creek passive pre suggested above would work, because it has a tape loop where you could put the X-Can (from tape out to X-Can input and then from the X-Can output jacks back to the preamps tape *in*). The Creek will take two sources, and it has a main out that could be used to drive a regular stereo amp at the same time.
Or, you could change the X-Cans for a preamp with headphone jack or a headphone amp that allowed more than one source (Sudgen, McCormack, HR Max, + ? ) ,...Melos sha, Audible Illusion L1, and Sonic Frontiers are just some of the pre's that would work. The Sonic Frontiers Line 1 would be a nice solution in that it's got tubes like the X-Cans, sounds as good (better) and uses the Headroom cross-feed circuit. In addition, you'd have a great remote and plenty of XLR and RCA in's and outs. The Melos might be found circa $400, the Line 1 $800 to $1100 (blow the budget ?)
You could try something like the (used $325/ $350 McCormack headphone amp) , which takes more than one source. Then you could ad a Musical Fidelity X-10D tubed buffer to the chain if you missed the tubish sound. Given that the X-Can's are going for $200+ used, the swap would cost you $150 ....less than adding the Creek (and the McCormack can, I think, drive a power amp as well)
Obvioulsy it's easier to solve the problem if money isn't an object (which it always is !) ,...but there's really only awkard or expensive ways around the fact that the X-Cans doesn't have 2 source inputs (which I would like to see on it).
As far as the tuner, ...I have a Marantz ST-17 ($350 used ?) and find it's very good (not as good as the Naim 01 I somewhat foolishly sold) ; it's ergonomicaly great and has full remote control and sounds good. But, I have not heard the 4000. I'd keep in mind that an antenna can make quite a difference, such that the 4000 + $100 invested in an outdoor antenna might well better a tuner that cost 4000+ $400 if used with a lesser antenna. Cambride Audio, Creek, Rega, NAD, Sony, Denon, Yamaha, Marantz ...all make nice tuners (and the list could be almost endless I'm guessing).
I agree with you that swapping interconnects would be a long term pain, ...but it might be OK for a short while (a year ?) ?