What freq.do tube amps loosen up in bass

Generally speaking, tubes aren't any good at putting out the current needed for good bass. It looks to me like you're crossing your sub over way too low(35Hz??). Unless your speakers are at or quite close to full range, go higher. Otherwise, you could be lacking base due to such a low setting your mains can't make up for. I must disagree completely with avguygeorge above. Don't bother looking at +/- freq. response of your speakers. That doesn't mean anything much unless you're system is in an anechoic chamber. Real world in-room response will vary much more, ESPECIALLY in the base. Timbre matching is also irrelevant here, since that takes place in much higher frequencies, and is more important in the midrange. Avguygeorge couldn't tell the timbral difference in a 40Hz tone, and neither can you or I. Also, that, "mud" he is talking about can happen at any frequency, so ignore that. Base is the hardest to get right usually because of room response. The reason that base is considered "omnidirectional" below about 80Hz or so is because the wavelengths are longer then the typical room, which means that the room becomes pressurized with the tone and can't be located("room lock"). The bottom line-a tube amp is probably just fine. Set the crossover just slightly above the freq. that your speakers start to roll off, then use test tones to adjust the volume. That way you'll get the best of both worlds; the tube amp can do it's job without having to work for all that base it can't do, and it'll sound better as well. Do the final adjustments by ear, but remember, if you get within +/-5dB, that would be considered quite good base results, unless you have a particularly large room.(over 40 feet long) I hope this makes sense to you; I'm writing quickly from work. Let me know if you would like more clarification. Good luck!