Hey aramism, welcome to the depot, nice to have you here
. We'll do our best to help with your issues. Some I can address now, some of them other members will be more help (e.g. the LED/siren issue, MJMP will hopefully be able to assist, it's not one I've come across before, and also may be an indication of why some of the other issues are happening ??).
Well it's no quite just 1 amp LoL although true say there's quite a bit on the MP1 preamp. I migrated to the newer MP2 when it came out years ago. We also get into all the other ADA gear (MB1 (bass preamp), MP1 Classic (between the MP1 and MP2), power amps, FX, cabs). But also all the other bibs and bobs various here use in their respective rigs and of course a fair bit of chat about guitars/basses.
The Boogie SPAX7s work well in MP1 but are very high gain. For what you are chasing (more clean stuff) you may want to consider other low noise 12AX7s that have a little less gain (have a read through some of the posts in the Tube discussion area for ideas). Unfortunately, while the ADA preamps lasted well for many years they are getting older and noisier (capacitors wear out over time and some other bits). There's a few MP1 noise mods which help, how are you with a soldering iron ? A good resource for mods and spares is MJMPs site (http://www.marshalljmpmodshop.net/index.htm) and he's also very good "under the hood".
OK so I can see from the pics you have the first MP1 version (vers 1) as yours has the instrument/line level switch (slide switch accessed on the top cover). This switches the A/B main outputs for inst/line level (and generally need a clean by this age with decent contact cleaner). I had the same unit initially, it's now owned by a friend (MikeB) and he's going through the refurbish process currently (so can also help no doubt). For recording you probably should switch it to line level (but depends on your recording inputs (obviously)). Also big tip, run it in stereo, sounds heaps better. MP1 were designed to plug into a poweramp (or guitar amp input (hence inst level switch)) and cabs. So direct into desk/record the patches are very toppy (for 12" speakers). Most here use a cabinet simulator for direct recording (or mike up the cabs). Plenty on that topic here.
IIRC the front input jacks are no longer available (and they mount to the pcb so not a simple swap like a "regular" guitar jack). The rear input on the V1s is line level so you could plug guitar into desk channel and use Aux send to feed it, if so use a monitor send (pre fader). Most here don't like the line level input and just want it the same as the front (for rear unit patching access in their rigs). MJMP has a mod which changes front jack to more normal non pcb mount style and replaces rear jack which is mounted on pcb and turns rear jack into inst level. This mod is a little bit tricky/fiddly depending on your experience with solder/desolder/pcb work (removing (desolder) and replacing the old jacks). But the instructions are easy to follow. MikeB and I (well mostly Mike) did this on his MP1 recently, took our time, all worked well.
They are a very gainy unit
but you should be able to clean it up, also you could try turning the guitar vol(s) down. Lately I've been turning my JP Les Paul vols down to 2.5 - 3 with my MP2 (which have way more gain than stock MP1 BTW). I suspect though there may be some other issues at play ?? not to worry, I'm sure we'll work it out. Obviously for clean tones you wouldn't normally use the distorted tube voice (changes eq in tube circuit and adds gain). SS and clean tube voices should be able to do what you are after. The SS switching issue is interesting, others will know more than me about why that may be happening. BTW have you got the manual for it (http://adadepot.com/index.php?topic=111.0), and if your EPROM isn't v2.01 (the last available upgrade) then grab one from MJMP and change that out (that's a very easy upgrade). The version is written on the sticker on top of it (EPROM located toward front of unit on transformer end) but also displays on power up.
If it were me I'd be doing (or getting done by decent tech) all the mod/upgrades that Mike has/is done/doing (and he's chosen to keep it stock sound, as is also your desire):
EPROM upgrade
Rear jack mod (replaces front as well so fixes that (BTW Mike had exactly the same issue with front jack)
MP1 noise mod (the one you need depends on if you upgrade the transformer with MDRT which also helps from noise perspective and highly recommended for performance).
MDRT ( Machinator Developed Replacement Transformer) very worthwhile doing but depends on your budget, not essential however (depending on your perspective (i.e. to me it's a no brainer/must do)
New tube board, MJMP can make you one that has modern/better components but is stock sound (rather than the gain mod versions)
Battery mod (this will dump all the non factory patches as it keeps the ram alive), from pics, you have original battery, it will die soon, you can do the battery mod or just buy a CR2032 that has the connectors attached/on it (they have them in electronics/hobby shops) which should last another 20 ish years, if you don't change the battery soon, you risk loosing all your custom patches, so write them down, you can also save them to PC and reload (but also write them down just in case).
New Vol pot
Replace all the rear jacks and pots (not essential but good idea).
You can go further and replace all the opp amps (that you can get modern new ones of). There's a post where one our members did this, bigger job to do and he did allot of research to get the best opp amps he could but he said the difference was astounding.
So this should bring it up to about as good as you can and stay stock sound wise.
And hopefully we can benefit from your production expertise/experience in the recording area, the more ideas the better
Cheers Richard