Efficacy- I also own the DB99mkII's and have used a few solid state amps on them so far. They work great, but you don't need to use an amp with that kind of power, from my experience the less power=more musical, this is not a concrete rule, but seems to be fairly constant. I had a very highly regarded pair of SS mono's and just didn't feel I needed the power and I could get away with much less power and even get back into tubes(which is what I did). With the quality of tube amps of today its a breeze to use them and they are very reliable. I have been looking for a low wattage SS amp with balanced inputs and its pretty hard to find- I want a second amp for summer/background and possibly movies, I can't see wasting tube life on back ground noise. I am thinking of getting a Pass Aleph 30, maybe not the best, but it should suffice for back ground music.
I have run amps as low as 30 watts and as high as 400, the results are typically the same. The speakers are very dynamic and can produce enjoyable sound at levels lower then 60dB, but are also capable of CRANKING if you desire. From my listening so far I find that tube amps make a more natural sound with the db99's. Just make sure you listen only after broken in, when new they sound pretty bad, the tweeter even honks a little when new!
Of course this is just my experience.....
