That's exactly the rig I have....Formula Pre and 20/20.
I've been using it via the slave outs into our standard PA, but I've also used it with a Hughes and Kettner 412 wired stereo 4 ohms, an Avatar 212 with vintage 30s, and an Egnater 212 with greenbacks.
I've been pleasantly surprised by any guitar I plug into this rig, from a 1960 Harmony H45 Mars with a single DeArmond pickup, to a Gibson SG with EMG active 81/85 combo.....Clean is clean enough to amplify an acoustic guitar well, and crunch is crunchy, OD is grind to grunge. OD2 is metallic enough to be a '68 camaro, steel wheels and all.
I really like it, and I've just tried it with a set of vintage Garnet (Winnipeg's Garnet Gillies, amp maker for Randy Bachman, Burton Cummings, etc) 4X8 cabs, filled with Marsland speakers. I can't believe the fantastic sound. Yeah, I know, I'm spoiled....but I love to play.