I have a Krell system and Thiel CS6 speakers and I had wondered the same thing - would there be any benefit from introducing tubes into the signal chain. I have tried three different options. Here is my experience with each:
Jolida Fox SS-X - I currently have this hooked up to my desktop system but I originally bought it for my full rig. First of all, it is not subtle. You either like what it does or you don't but at least in my system it has an unmistakable sonic signature. I pretty quickly decided that I didn't like it and I moved it to my desktop system where it works really well. The soundstage control is very nice with nearfield listening and the tube effect compliments this system (Denon integrated, Polk desktop speakers, Topping D50s DAC). I've put it back in my main system a few times and I always came away with the observation that it is interesting but it reduces the definition in the bass and it adds some darkness to the lower treble which aren't complimentary to the sound. In my desktop system it's just what the doctor ordered.
Yaqin SD CD-3 Tube Buffer - I use this in the tape loop of my Krell KRC-2 preamp (amp is a KSA 300S). Unlike the Jolida, the effect of this unit is very subtle. It's easy to switch it in and out of the signal path and it took me a while to detect any difference. The best way I can describe the effect is that it adds a little sparkle to the high frequencies. I like the effect (it certainly does no harm in my system) and I leave it in the signal path all the time. I bought a pair of fancy tubes but I haven't rolled them yet.
Black Ice (Jolida) Glass FX Tube DAC III W - A similar unit won a listening competition in Arizona against a bunch of well reviewed DACS and for the price ($700) I decided to give it a chance. I have a PSA PerfectWave MkII DAC, a PSA PW Transport, a Marantz SA 8005 (SACD) and a Krell CD 250/2 (HDCD) player, and a vintage Resolution Audio DAC (HDCD). My experience is that the SQ difference between modern DACS and players is pretty subtle and the Glass FX fits that pattern. Three of my units (PW MK II DAC, SA 8005 player, and Glass FX) sound so similar that I would have trouble telling them apart in a blind test. Depending on the recording, I think I can hear a little better soundstage depth and articulation with the Glass FX but it's something you really have to listen for. I also bought a pair of fancy tubes for this unit and I haven't tried them yet.
My recommendation would be to play around with some inexpensive alternatives before dropping a lot of money on a tube preamp. You can also pick up an older unit like a ARC preamp that is known to have an obvious tube character and see if you like it.
