Tube preamp output impedance at low frequencies


I'm looking to use a tube preamp with an active crossover (so as to send low frequencies to a sub), and unfortunately, most active crossovers have rather low input impedances (10k-20k). The only exception seems to be the Pass Labs XVR1, which unfortunately is out of my price range.

This being the case, I want to get a preamp with sufficiently low output impedance, to preserve the bass in my system. I know most manufacturers quote a single number for the output impedance, often at 1000 Hz, and this can differ greatly from the output impedance at 20 Hz. An example is this unit, which has Zout of 415 ohms at 1000 Hz, and 4.8k ohms at 20 Hz.

http://www.stereophile.com/tubepreamps/208bat/index4.html

I assume it would be a very bad idea to use this tube pre with one of the crossovers I'm considering. Other than Stereophile, is there any other source for tube preamp output impedances across the whole frequency domain? If I email a manufacturer directly, do they usually have this info on hand (and if so, do they have it for discontinued models)?

Has anyone else also faced this problem? If so, I'd love to hear about your experiences.
rrolack
I believe most subs should have a -3db rating on the high end of their frequency range (e.g. -3db @ 150 Hz), this might help in finding an appropriate unit.
What are some other good tube preamps with low output impedance at low frequencies, preferably balanced? I know the Audio Research SP16 is very low, but unfortunately it is not balanced. Price limit is $3000 new/used.
Look for something that's transformer coupled at the output. My Supratek Chenin has no problems driving Gilmore Raptor amps with 8K input impedance. Plenty of good tight bass.
Nemesis,

This one is not balanced, but it is what I'm moving to now: Lamm LL2. Has 250 ohm nominal output impedance for mids/trebble, and goes up to 3300 ohm at 20 Hz. You can check out their website for the info, or see Stereophile measurements. They run used in the $2.5-3.3k range.

Check it out, though, as its ergonomics aren't for everybody. No bells and whistles whatsoever.

Horacio