Preamp/Subwoofer connection question....

Most subs will allow you to connect via speaker cables from your amp, and there are different ways this can work. Infinity's usually run speaker cables from the amp to the sub and then to the main speakers, and filter lows off from the main speakers (unswitchably.) REL gives you a proprietary cable with connector which runs from the same speaker binding posts that your main speakers' cables do. The RELs supplement the main speakers in this hookup method, without keeping the lows from reaching the main speakers. Vandersteen subs _only_ hookup via speaker binding posts, unlike the other two examples (as well as most subs) the Vandersteen models give no RCA/line level option. You could see the hookup box and filter theory notes at the Vandersteen web site; it's not like anything else.

Finding a decent ss pre in that price range is hard, period. I don't see why a pre can't be tube - the voltages are so low you avoid most of the drawbacks of tubes. For examply, if you can live with the service risk, norh makes a nice, cheap tube pre with pre outs. So does Antique Sound Labs. Of course, if you're asking for a preamp for use with a sub via RCA/line level out it's better if the preamp has two pairs of pre outs.

There are passive preamps which fill your requirements, and they might sound good. For example, redwineaudio.com.

IRD has a decent pre with sub jacks, but it's $150 or $200 more than your stated limit.

I'm sure there's no limit to suggestions, but this will get you started.

Phasecorrect, some to many preamps these days have two sets of main outputs. I use the pre's XLR main outs to the amp. When I did own a subwoofer, I used the single-ended main outs for the sub.

As for the price range you mention, I can't help.

-IMO

B&K PT-3 and above has a sub out and crossover.

If it's good enough for you, the NAD c160 has two pre outs. I have it , and I like it. It can be had in your price range.

You can try a digital crossover like the Behringer DCX2496 ($350) or the DBX Driverack ($499), which is what I have. You will be able to adjust both the high pass and low pass settings between your speakers and sub to get the best from both. You can bi-amp your speakers with another amp, if you want. Also, there is a seperate microphone available that you can use to EQ your room.
One of the problems in using just a pre with multiple outputs is that there is no crossover to stop the speakers from trying to play the lower frequencies although usually the sub has one so it will not play high freq. I learned this the hard way, I blew out a driver while playing a movie with some really loud low freq. sounds.
These digital crossovers are balanced which makes them ideal for fully balanced systems. But they can be used single ended if you get XLR to RCA adapters.
Sonny