Warm and mushy, eh? Well, I think I may know what you mean, though they aren't exactly a very flattering pair of adjectives. Tubes are probably a very good thing just in terms of warmth. Laid back in nearfield is a tough one and may have more to do with the speakers you choose than anything else. I'd go with a tubed integrated amp, like a Jolida 102B, Sophia SET Baby (disclaimer: I am selling one of the latter), or other EL84 based small integrated. This should run you from $350-450. Tube buffer stages are not really necessary (I assume you are talking about a DAC buffer). Just get a tubed DAC. I would recommend an MHDT Paradisea (has one tube in the output stage for that warmth and, um, mushiness you are after). Actually the Paradisea is a very natural sounding DAC...I wouldn't really call it "mushy" at all, but I think it will fit what you're after in context. These go pretty fast, but if you are patient and vigilant you should be able to find one for about $400. I would not use a soundcard - an external DAC will give you better results IMO. Ideally you want to take that conversion outside of the noisy electronic environment of a computer chasis. OK, I've spent 800 bucks of your 1k...that only leaves 200 for speakers. There were a pair of Omega Compact 3i's up for sale here that would probably be a nice match with a low-power tube amp in nearfield. I think they were around $400 though. You'll need something efficient if you go with the tube amps I suggested. There's a Klipsch monitor that I have not heard that is supposed to be very good. I like their older stuff when Paul still had a hand in things, but haven't really heard that much of the new stuff that floats my boat, but their RB5 monitors are supposed to be very good. I've seen them for as little as $275 on the used market. I don't know if they'd be laid back or "mushy" though. I'll defer to someone else on that one. I would say the general sense I get from your description points to tubes. I'd try to find some better adjectives though :-)
Have fun!
Have fun!