Of course it can be done.
I'm wondering, what's the NEW piece of this puzzle? New dbx gear?
The preamp has two tape loops plus a processor loop (essentially 3 tape loops). You can discount the tuner for this discussion as it already has a dedicated port.
Here's what I would do (and have done *). Get yourself a dbx 400 or dbx 400x Program Route Selector and simplify all of your current and future analog hookup options.
(* I, too, have a CD recorder and a tape deck plus some different dbx units (4bx, 110x-ds; 224) and other toys.)
One on eBay at $59 (a steal so far) expiring Mar 9:
and a link to the manual so you can see how it works:
It takes up ONE tape loop and allows attachment of 3 processors and 3 tape decks PLUS a dbx-style noise reduction system if you have one; and can logically rearrange the processors to be in line with any source and BEFORE, AFTER or BETWEEN tape decks with or without the Noise Reduction Processor involved, all via front panel illuminated buttons.
If you don't want to go the 400x way your options are severely limited re: the tape deck because even though the dbx processors can fit in nicely on the preamp, I don't think the preamp can place them in front of AND after the tape decks, depending on your desires for pre- or post-processing.