How long tubes last in a circuit will depend both on the maker of the tube as well as the circuit design. I've known McIntosh tube amp owners who have left their amps on for years continuously with hardly any wear on the output tubes. My ARC Classic 150's lasted me 5 years on a set of Philips 6550 tubes, where the same vintage tubes barely went for more than 15 months in my Jadis JA80s. But NOS Gold Lion KT88s in the Jadis have gone over 3-1/2 years with very little wear. So no hard and fast rules here.
Of the brands out there now of which I have knowledge, the VTL and ARC designs, as well as Macs, seem to be easy on their tubes; I'm not that familiar with how long tubes last in VAC or CJ products, I'm sure others can comment here.
As far as power goes for your 804s, the only way to really know is to try them out, but if you're not after the deepest bass (and you won't get it out of the 804s, anyway), and you don't have a large room or listen too much at 90+db levels, you'll be surprised how well tubes will drive your speakers. Some of the best sound I ever heard from a pair of Dunlavy SC-Vs came from a VAC Renaissance 30/30 (it wasn't organ music or earthshaking volume levels, admittedly, but certainly a normal listening level), and I remember years ago hearing a pair of B&W 801 Matrix speakers being driven beautifully by Kebschull 35 watt tube monoblocs. It would pay to give some of the manufacturers, like VAC as noted above, CJ or VTL a call and see what they think they have which will drive your speakers (I'm sure ARC's 100+ watt designs would work well on them, but I consider them expensive amps). Good luck--I think given your musical tastes you'd like tubed amplifiers with your speakers.
