Preamp Suggestions with LFE output

I have an old Audio Research LS3 with two sets of outputs.  If I drive my sub with one and the other drives my Threshold S/200, the S/200 is way more sensitive than any sub I have tried.  The only way I was able to use the sub is to put a couple of RCA 6DB pads in the input to the Threshold.  while it balances out the sub nice it kills my highs.
I really want to move to a preamp with LFE outs with a level adjust for that output.  TI can go with a number of surround processors, but it is a dedicated listening room and that a whole lot of crap I dont need.  I guess I could switch to an amp with a level adjust on the inputs but havent seen any of those in this world. 
Your subwoofer does not have crossover and volume control? I recommend getting  an audio subwoofer. Your system looks worthy of it. I would not trade in an Audio Research piece of gear unless I was upgrading to another one. I don’t know any high end audio preamps with an LFE. That is in a different realm, home theater. If you want good sounding audio for music, you want a subwoofer designed for it. .

Unless you are converting to a home theater system... in which case you will likely be severely compromising your two channel audio. If that is what you want... that is ok.
Audio subwoofers have sophisticated adjustable cross over and volume control as a minimum. This allows the sound to be carefully feathered in with the main speakers so not to create a huge bump in volume where they overlap. subwoofers like REL come with very sophisticated equalization capability.

Sub woofers made for relatively inexpensive home theater or gaming have little or no control and are there to make explosions loud and just add bottom. These are attached to home theater LFE ports. As I mentioned, you don’t want to add this to your system. It is already way above this level of performance,
Thank you all for the responses.  Yes my sub has a volume control and a crossover.  Both of my subs do.  Watch has the same issue. My AR preamp drives my Threshold amp into ear splitting volume long before either sub even gets started.  The threshold amp requires a fraction of the signal level than do either sub.