Both my 5/5 accordions have a forward bias, that's because they 29 and 34 pounds... lol
Kidding aside, I don't find this accordion (the Imperator) with an obvious forward weight bias any more than my Elka or Iorio. This may be due to the slight weight savings caused by the metal case design.. My Hohner Morino VI N... yes a massive difference forward bias feel in comparison. It's just made to be used sitting down, but that's the nature of this kind of beast. Take a bow with that thing, and you are going to topple forward. .
Not featherweights but not the very heaviest either. My Hohner Artiste XS comes in at 39,5 pounds (18kg). What's important is that your upper legs are horizontal so the accordion can sit flat on it (not with a corner biting into your leg) and then the weight is not a problem.
If an accordion feels like it wants to topple over, forwards, it means your chair is too high, causing your leg to not be level to support the whole bottom of the accordion while it is upright (straight up).
I pretty much never stand in playing position with the accordions anymore except to get to a chair, and it's a bit of a habit of mine to have the left thigh as close to horizontal so that there is no slipping forward (as one can see on my videos), but that forward bias is easy to feel based on where the pressure is located on the thighs when sitting (closer to the hip or closer to the the knee).
The Imperator is definitely heaviest closer to the hip (about 1/3rd the way from the hip towards the knee) and the Morino is heaviest a bit past the 1/2 of the distance between hip and knee, that's even with the accordion so close that the bellows are touching by body. Besides width differences, weight bias is the biggest factor. The reality is that this is only about 3-4 inches, but it has a completely different feel. It's really neat to feel those differences by moving back and forth between the accordions a few times.
A video of me testing out the accordion coming probably this weekend!
The weight distribution of an accordion depends on its internal construction: where are which reed blocks located, especially on the bass side, and how heavy is the bass mechanics and where is most of that located... The straps of course make the biggest difference. The "Gardinenstange" of the Morino lets the accordion lean forward more than strap brackets mounted on top of the accordion.
That is quite common in 5 voice instruments. Not including a mute there are 31 possible registers, so to end up with a manageable number every register with tremolo includes the M+ and a register with M- without M+ then does not exist. Even then you still have many registers.
An alternative solution is to just have a switch (a lever) to turn M- on or off. You can then have a large selection of registers and then double it by having each register with or without M- included.