preamp with two adjustable outputs??

I dont know of any that have seperate output levels via 2 output jacks but you can just buy or make attenuators to choke a strong signal or buy amps with adjustable gain. But in all honesty a premium quality crossover or matched amps is always the best bet, I run 2 matched amps with outboard crossovers and at times ran unmatched amps but had gain on atleast one unit.

McIntosh c220 & C2300

The big Mark Levinson

If you can attenuate the signal at the sub (most do) you should be able to match them to the mains and then as long as both outputs are ganged together, you will be all set w any pre w dual outputs.

The "Supratek Cabernet Dual" does exactly what you're wanting. There is a excellent review on 6moons.

Btw, you might be able to pick one up second hand