Yes, I think for how well built they are they are priced very fairly! I have heard only good things. The diy 51x does seem to have issues that the big version doesn't, but I imagine as you are saying it is the different (smaller?) transformers made to fit in the modular units.There is no bass roll off in the two g9 units I put together (unless you engage the HPF, obviously) but these are not the 500 series variant which uses different transformers.
If you are not building these yourself then Jakob’s commercial units are very reasonably priced for the parts, labour and quality assurance. I think they sound very good.
Do you know, if it might be possible to make a G9 that is 4 channels in 2 rack units, by moving the power supply to a separate unit? It would make this much more possible, as we are mobile and already 2 rack units is a lot of space for how light I'm trying to get a location recording rig.