I don't know exactly what you're after, but from a "studio quality" perspective there are a few companies I've found that make "high-end" products at mid-level prices. Two that immediately come to mind are Ashly and Symetrix. I've tested products from both of those companies from a technical spec perspective (I'm an electrical engineer) and was very surprised at the level of performance I saw from both of them. If you can get a Symetrix parametic EQ at a decent price I think you'll be more than pleased (and Ashly compressors are top-notch).
But really, making a good EQ really isn't rocket science. I think any reliably piece of equipment would be fine for teaming up with a guitar amp/preamp. Don't forget, most guitar amp tone controls are just cheap carbon pots and carbon film resistors - it's not like they're "studio grade" circuits in the first place.
Back in the day, it was a common "trick" to pair the MP-1 up with a Furman PQ series parametric EQ. The Furmans were fairly poor performers in terms of noise and technical spec but they had a crazy amount of gain that could overdrive a preamp hard. But that's up front, not in the loop.
For something in the loop, I'd go with a para EQ from a reputable manufacturer - like I said, it's not rocket science and doesn't necessarily have to be "studio grade".
The only time I'd get particular about really high-end level specs is if you're recording and/or not using a noise suppressor. On the studio side of things I'd start looking at studio level equipment, otherwise, I'd go for something well-built and roadworthy.
Having said all that, I do like the Ashly 551E parametric EQ - studio grade for pretty cheap.