You ought to include the Adcom GFP-565 preamp on your list, since it was (and still is) an excellent preamp. There have been several GFP-565's for sale recently on Audiogon for around $300.
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Passive F.T. audio volume control is said to be a great passive for $500 new and about $300 used. Any of the passives should work for you in that system and sound much better than active pre in your price range. The F.T.Audio has second set of outputs you can use for your sub and has four inputs for source. I saw a company on e-bay that sells a one input/output passive for under $200 a few weeks back, can't remember what brand name though, you may want to check over there under preamps.
As I said, I had a Adcom GFP-565 preamp and I was disappointed in the sound. Truly mediocre. Switching to other pre-amps revealed alot of detail that I was missing, and really opened up the soundstage. I see no reason to go back to something that was disappointing, and whose failings I know. Time to see what some other peices can do.