My current studio monitors are Tannoy System 800's (passive). I drive them with the power section of a Kenwood Ka-8100 (top flight amp from the early 80's I got for free years ago - they don't make them like that anymore).
The Tannoys are extremely clean and accurate speakers. The manual warns its easy to turn them up loud without noticing it (due to their very low distortion figures) and indeed this is true. My mixes often sound better on other systems than these units because they are brutally honest and quite 'unexciting' or reserved - however, vocals and sweet detailed sounds when well recorded blossom in front of you.
So guess I'm a Tannoy fan!
However, they do tend to barf with large amounts of low frequency material so I set about sorting out a sub for them. I got a 10 inch Dayton Audio reference series sub driver from parts express and set it up in a bass guitar cab I had lying around, hooked up to a Yorkville 2000watt audio pro power amp and a dbx crossover. The dayton sadly did not fill expectations - though it handled the formidable output of the yorkville it was not capable of clean bass even a moderate levels - there was very audible distortion. I'd estimate one would need 3 or 4 of these 10inch speakers to achieve clean output at say 100db.
So I sent that back and just hooked up an 18inch carvin PA sub I use in my bass rig - a world of difference. Clean deep effortless lowend. I was expecting the opposite situation. One might think that PA gear is 'lowfi' or not appropriate for a hifi reproduction situation but this is just not the case. Big speakers (at least with direct radiator designs) and powerful amps are the key to good low end even at 'normal' listening levels - I'd noticed even at quiet settings my bass rig always sounded much more authoritative than any hifi at the same volume level. I'll get an RTA mic at some point at calibrate the whole setup properly. Standing waves are always a problem but it should be possible to EQ out the dips in the lower octaves at the mix position.