Tube preamp output impedance at low frequencies

Second the Quicksilver Full preamp. It will actually drive K-Horns without a power amp--not that I am recommending that, but it will drive any load you connect to it.

As long as you stay below 1000 ohm output you should be okay with any of them. However it is hard to find many tubes with that low. But there are a few, BAT makes some in the 500 to 700 range. And Conrad Johnson has some as low as 50 ohm output which would drive a super low 10k input on a crossover. However if you can use XLR balanced connection, with like a BAT or similar normally the crossover or amp driven will be for example 20k input on single ended, now 40k input on balanced connections, thats why most of these units are mostly for PRO audio use not really home use.

Yes your bass will suffer using single ended RCA connections with most of the Tube preamps that are 1.5k ohm and up outputs unless you can get an easy load of 20k or better on the crossover(or amp connected) to the other end of it in my experience. Highly suggest trying to go XLR in this case if you have to use a PRO unit in the crossover.

By the way there is a connection "Impedance" convertor device made by Rolls which is a pro company that can accept your RCA input off the Preamp, and convert it to the XLR output with an actual gain boosting circuit, I used this in a pseudo style Pro home theater setup once with good results feeding stereo subs, as it does have a right and left channel, also its pretty cheap around 40 bucks online. Again this is only if you must go this route, I don't suggest pushing any of this kind of setup if possible.

The output impedance of a preamp at low frequencies has a close relationship to the size of the coupling capacitor at the output of the preamp. Many tube preamps including the Quicksilver (which has a very low output impedance otherwise) have a rising impedance at low frequencies due to the output coupling cap.

Whether this is actually a problem is another matter. The interaction of the coupling cap and the input impedance of what you are trying to drive follows a formula:f= 1,000,000/C x R x 6.28 where
C= the value of the coupling cap in microfarads
R= the input impedance in ohms
f= minus 3db point in cycles per second

So if you have a 5 uf cap and a 10K input impedance the cutoff frequency is 3.16 cycles per second. Keep in mind that the rolloff will manifest at a frequency 10 times higher, so you will notice a loss beginning at about 30Hz. This is why it is so important to get subsonic bass response from a preamp!

There is a tension between the bass cutoff and the overall sound of the preamp. The bigger you make the cap, the better the low end, but at a sacrifice of transparency.

There are tube preamps that bypass this issue by use of a direct-coupled output.

Sorry to disagree with Atmasphere, but the Quicksilver Full preamp does not suffer the usaul rise in impedance at low frequencies. The output coupling cap is 160 mfd and there is 18 db of feedback from the totem pole cathode follower to the preceding gain stage. Yes, yes that evil feedback! But in this case it is around a single gain stage and it is not limited in the low frequency range given the large value coupling cap. All I know is that it works and works well. The imaging of the preamp is phenomenal, which I attribute, in part, to the -3db point being well below .1 HZ. A very stiff power supply and high quality parts also contribute to the result.