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.
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.