Hey scsi, well definitely could do with new tubes and unless the rear jack has been modded (I suspect it hasn't) the rear input is line level, so not a place to plug in the guitar directly.
Sending it to MJMP for a revival/service is a good idea. I have some ideas re the sound, just guessing and thinking out loud... I think the front input jack may be worn (as I said before, it takes the majority of the wear and tear over the years). So rear jack mod will fix that (and at this age it should be replaced anyway). My reasoning is there may not be enough signal hence thin nasal tones. So you reloaded the factory settings ? (1 to 29), the others (30 to 128) are all user slots and who knows what the previous owner(s) put there
if anything (hence blank patches). But you can also overwrite 1 to 29 with whatever you want, I'm assuming right now they are the factory defaults since you did a factory reset which takes the 29 patches from the EPROM and overwrites the user 1 to 29 slots.
So lets try to get patch 1 (Marshall patch) going/tested. This patch should sound good and full, so try it and let me know how it goes.
So a nasal sound (while it could be lots of things) could be eq settings (but the eq on the factory presets are fine so patch 1 should be good from that perspective). The flanging you get on the other patch could be chorus settings ?, also there are trim pots for the OD1 and OD2 tube circuits, if they've been changed it could cause gain issues.
Ok just re-read your post and looked up the furman parametric and the Fireface (which is a digital I/O interface for PC recording). So (pls correct me if I'm wrong) you are monitoring through a studio style setup ie going direct from MP1 to I/O to PC ? and then listening through full range speakers so no wonder it sounds nasally
there's way too much top end because the speakers are full range.
The MP1 is designed as a stage preamp to plug into a clean/transparent stereo poweramp driving 2 (or a stereo) guitar cab(s) loaded with 12" guitar speakers (eg 2 x Marshall quad boxes). All the MP1 patches have the right balance of treble to sound good through a 12" cone (not into the tweeters in full range monitors) so you end up with wayyy too much treble going direct. To go direct with MP1 most use a cabinet simulator which, apart from other things, basically cuts back the excess treble (eg ADA GCS3, Micro cab etc) to sound the same (similar) to what the cabs sound like.
Now interestingly, the headphone out on the MP1 has a very simple "cab sim" style filter to remove the excess treble so it sounds ok in the headphones (which are also full range). It's not as elaborate and versatile as a separate cab sim but if you run a R/T/S to 2 x T/S lead (stereo jack to 2 x mono jacks (same as an insert lead)) from the stereo headphone out jack to 2 channels on your Fireface (pan one hard left the other hard right) it should sound much better (not as good as power amp and cabs
or as good as decent cab sims) but usable for now.
If you are going to use the PQ3 (personally I don't think you'll need it) I wouldn't put it in front of the MP1, hey you can but not good gain structure, let the MP1 input just get straight guitar. You could use the PQ3 in the FX loop, as PQ3 is mono it's not so good after the MP1 (MP1 should be run stereo if you can). The PQ3 is also a preamp so you could run it into the rear line level input, it will have too much gain to run into the front input.