Fostex and Lowthers have a very narrow set of strengths and you give up a lot of other areas. Those that like them believe they have excellent midrange and imaging focus. However they have limited low end, truncated high end and so tend to sound very nasal to my ear. They're not for everyone. In fact I think they're very unique and probably for a very few.
I agree. The Lowthers I've heard have a nasty upper midrange peak that makes them sound "broadcast-y" (for lack of a better term). Sort of like how some Klipsch models tend to sound "shouty."
You can tame all these resonances with various circuits in the crossover, but each piece of EQ added there reduces sensitivity, which makes the amp work harder (more watts and current drive capacity required from the amp).
Just look for speakers with relatively high efficiency that do not have extreme impedance dips.
Unfortunately, most Klipsch speaker systems have quite irregular impedance curves, which makes them sound strange when driven by single-ended tube amps with no global negative feedback.
There are some moderately sensitive (about 90dB for 1W at 1m) cone-and-dome (direct radiator) speakers that have fairly benign impedance curves, and therefore sound quite good driven from SE tube amps. However, with only 3 watts or so on tap, they won't go very loud before the amp distorts too much.
You might want to talk to Dave at Planet10 audio, and Wayne Parham at pi Speakers. Both of them are quite familiar with this particular design problem.
Open baffle designs like the Wild Burro Betty might be a good alternative. They make a room-filling sound with less power required from the amplifier. Unfortunately, open baffle speaker systems need to be extremely large to get any bass out of them.
Probably the best solution would be to bi-amp. Use an active crossover to send lows to a high power amp and a pair of woofers, and send the higher frequencies only to the SE tube amp and whatever speakers you choose. Since it's the low frequencies which require high power, you can use your low power SET amp as a 'mid-tweeter' amp, but with a lower than normal crossover of between 80 to 300 Hz (depending on whether you use one sub to cover both channels or one woofer for each channel). If you use a pair of full-range speakers for the SET amp, and (sub)woofer(s) with appropriate high power amp(s) for the lows, you'll have what some hobbyists are calling a "FAST" ("fullrange and subwoofer technology" -- ok, it's a dumb acronym).
A really good single-ended tube amp can have beautiful mid and high frequency reproduction, but will usually have very wimpy bass drive capabilities. It's probably best to let your SET amp work at what it does best, and not try to make it do what it's not capable of doing (bass).
Anyway, that's just my opinion. My personal compromise has been to use a push-pull amp with really good (expensive) output transformers and 2A3 triodes, so that I get usefully low distortion with no global negative feedback loop around the amp, and still have a reasonably low output resistance for driving real-world speaker systems. The amp still chokes on most commercial speakers, though. For instance, it cannot drive bookshelf-sized B&W speakers well at all. It does OK with Tannoy speakers, but you can hear that it doesn't have a really firm grip on them (if you know what I mean). It also has a hard time driving a pair of Alpair 10's in small bass-reflex boxes. The Alpair drivers sound more 'alive' when driven from my home theater receiver, believe it or not.
PS - I do think MarkAudio Alpair drivers are awfully nice.Edited by rongon - 9/18/13 at 9:17am