Ok, ill try. I have a D800 and Subway 212, a GK MB212 and still have my Rumble 500h and 210(now my sons rig

so i can compare all three.
Its a bit difficult to explain, but Mesa is build in the USA from all top quality components, the best drivers they can find, etc, which is why they cost so much. However, as a result they sound fantastic, the notes have more "weight" and thickness, while still being super articulate, clear and punchy. Mesa has Mids to spare, definitely the best rig I've ever played through and lives up to the hype. The D800 has a super simple eq, quick and easy to get a great tone, set flat, plug in and go. No fiddling with 15 knobs. The 212 is beastly, a bit larger than other 212's but still weighs around 50 lbs, so still very manageable. It's a very tight and articulate box...mids in spades across the spectrum, and of course super tight and thick lows without any boominess or mud.
The GK sounds very good, has the "GK" sound and is a solid second, still not quite the "presence" or magic mids of the Mesa. The MB combo is my only GK reference and they have many other lines. But for a combo it checks all the boxes, lightweight, portable, but capable of thunderous volume. Tone wise it has a nice tight sound with a bit of an edge in the highs, a bump in the low and low/mids and a bit scooped in the upper mids.
The Rumble is more of a "budget" rig by comparison, but for the money hard to beat, sounds solid and has a reputation for being very reliable, a working mans "model-T" type rig. However, it's not a fair or even comparison to the Mesa or GK. The Rumble is reminiscent of a classic "bassman", very open and bloomy lows, and IMO a low low/mid forward rig that can get a bit muddy if your not used to the eq. I find myself dialing the lows and low mids down a bit most of the time and bumping up the upper mids. With a P bass the Rumble really dials in a great classic "Fender Bass" tone, again for the budget conscious, its one of the best options.