Arcamera - I owned and loved a woo 3 for many years. It was a great match with my hd650's but (IMO) just isn't detailed, resolved, dynamic or fast enough to really show off what the hd800's can do. Tube collecting and rolling can be a lot of fun, but I question investing several hundreds of dollars on rare nos tubes for an amp that is only worth about $350 used. Over the years, I acquired most of the highly recommended tubes for my woo 3 - holland pq 7308's and pq 6922's [drivers] and various 5998's and 7236 [power tubes] etc. (all purchased back when they were not so obscenely overpriced). No combination of the above made the woo 3 outstanding with hd800's. I have since acquired a GEC 6as7g, as discussed above. [FYI this is a british made tube, not a product of general electric.] It is a very nice tube, but again, we are talking about subtle changes in the operation of the amp. In fact, even skylab - head-fi's most venerated proponent of the GEC tube (in the past) advises
against buying them now given the outrageous pricing. Unless you are going to undertake serious mods, like replacing all the cheap electrolytics in the woo and GrindingThud's awesome implementation of a bottlehead-derivative constant current board (
http://www.head-fi.org/t/94853/woo3-modified/225#post_9194972 ), my advice, FWIW, would be to look at a different amp.
Investing in a GEC 6as7g and a pair of decent 7308's probably would warm up the sound a bit. But for the same money, you could build a bottlehead crack kit w/ speedball that pairs beautifully with the hd800's and betters the woo 3 in just about every way. Frankly, I prefer my modded crack over everything in the woo lineup short of the WA5, and I'm not the only one who feels that way. (See, e.g., post# 2573 in
http://www.head-fi.org/t/476650/crack-bottlehead-otl/2565# ). As a bonus, the guys at bottlehead, and the community surrounding the crack (both here and on the bottlehead forums), are awesome, and at the end of working through the step by step, paint by numbers styled assembly guide, you will know more about headphone audio than 90+% of the members on this site. Basically, the build takes ~10 hours for a first time builder (like I was), and is an awesome learning experience that culminates in a great tube amp.
Alternatively, if you simply want to warm up the sound without paying much of anything - try out the anax mod. This article explains it and provides a video tutorial at the end:
http://www.innerfidelity.com/content/diy-modification-sennheiser-hd-800-anaxilus-mod 
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