Man that's great information and advice Bro - it took me 5 MP-1s to get the right one and had I not been on this forum I would still be burned, the last one is still due to arrive tonight.
And one thing I will add to what you have said is this; after speaking to all but one of the sellers I realized that they did not have a clue about the ADA MP-1!! They were in innocent in the fact that there are not many of us are playing these and in my case, none really knew how the unit was supposed to work or sound!
So I cherish our Brotherhood on here Folks!
There are additional considerations we could do, bro.
Sometimes people is honest and other times people is not.
You cannot ever be 100% sure.
Sometimes people who sells stuff is not aware about what they are selling: they're (re)sellers, not users.
We also sell stuff, but we're users first.
And it's different.
When I sell something, I know everything of that item.
You can ask me everything you want and I will always answer.
When I started looking for a MP-1, can you imagine how many people was not able to answer the simple question "what's firmware version"?
It's not so difficult to answer, even if you don't know it: you only have to power up the unit.
But you must be an user to know this little secret. ^^
Well, I discarded MP-1s sold by people like that.
If they do not even power up the stuff they sell...how the hell can they know if it works fine or has issues or how it works?
They buy, they sell and that's it.
Naaaaaaaaaaaah.
Usually I also discard shops that act in that way.
I found an used MP-1 in a shop near here...200 euros...they said it was in perfect conditions.
Perfect conditions = scratchy output level pot, very noisy unit, scratches everywhere, factory reset not performed, probably dead battery...and who knows what else.
I left it in the shop.
Then, as I wrote, I also discarded modified stuff...for many reasons:
1) the seller could have bought the unit modified from someone else with bad skills
2) the seller could have modified the unit himself with bad skills
3) the seller could be not aware about modifications and/or what they do
4) the seller could be not honest
There's a modified MP-1 near here...and I also contacted the seller many times, just to know.
He always answered after days and days.
At the end I gave up and bought another one (lucky me) and he wrote back too late (for him).
Well, when I bought my old 1.38 from a friend...it was my first rack preamp...it was more then 20 years ago...I wasn't aware about a lot of stuff...
Today, after all these years, I suspect that MP-1 had some issues, as I wrote...but I also suspect that a friend wouldn't have fooled me and probably he wasn't aware about those issues as many other people at that time.
We had not The Depot and people like these guys!
Anyway, he used that MP-1 for long time and I used it for long time after him.
Maybe it could have sounded better, but it didn't sound crappy at all...just a matter of gain and maybe of input, as today we know very well.
I have that MP-1 no more but I still have that friend.
We're also speaking about "old" stuff.
If you buy an original 1959 Marshall SLP...well...I bet it would have issues and you would take it to a good tech guy.
Of course, the value of a 1959 SLP is different and would worth the risk...
Very often, after being a non-expert guy in the past, today I ask myself how much fun is to still buy this stuff.
The answer is that's a lot of fun.
I just teached myself to deal with people and attempt to recognize a fooler.
I've to say that I did not meet too many crooks in these years of selling and buying guitar stuff, but I recognized some of them just in time.
Luckily, if I understood right, you had your money back for those 4 "broken" MP-1s and you sent them back.
That's the good thing.
Now you found a keeper and this is the 2nd good thing.
Let us know about the last incoming MP-1!!!