If you are mostly line level then you don't need a preamp, and if you have all the eq and comp ITB you need, I'm not sure what you are after. Color? The Newton has all the things you said you didn't need: preamp, eq and comp. It will add a type of analog flavor, but I wouldn't spend that kind of cash just for box tone unless I was loaded.
If you are just after saturation and a little bit of dirt, like rough the signal up a bit and add harmonics, I would be looking at things like a pair of 542 or a stereo Density. You could search tape emulator type effects, saturation, and see what people like.
BTW, have you tried comparing a mic signal from your amp speaker versus the "direct out" on your amp? I've used the DO a couple times on a couple amps and it sounded like ass. I could get away with it on a bass amp, but not on the guitar amps I tried it on (for live PA).
The bigwig sausages will wander by and be more help.