preamp with two adjustable outputs??

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.

by not adjustable, do you mean frequency roll off or volume or both?

If you are saying that there is no way to adjust the crossover frequency at the sub, then you are going to have a problem w/o an external crossover. There will be overlap unless the natural roll-off or low pass frequency of the sub mates with the natural roll-off of the mains. Its possible, but unlikely. Another option would be to use an HT/SSP that has bass management as a pre-amp, OR get a Velodyne SMS that will function as a parametric EQ and RTA to get the bass matched btwn the bottom of the mains and the top of the subs.

"MBL 6010D has output adjustments for left and right channels for each of 2 main outputs"

Well that certainly complies with the OP's request for a "high end" pre-amp!!!