IMHO I do agree to a certain extent. I own and use/have used a Rumble 500 quite a lot. The RB115 is more soft (weak) and you may say it’s from lack of upper mid.
But… they also react different to different basses and the RB is not muddy with a P-bass with flats, it’s actually sounds very good all of a sudden. Also the RB being more shy as far as output gain goes you need to turn the master volume up (no gain control here) to make it ‘wake up’. But just as it’s predecessors, it sounds better the harder you make it work and is actually usable almost all the way up the whole volume spectrum.
Both combos are good in their own way with the RB115 being more vintage traditional and you may say not as clear (or brite) as the Rumble that is very powerful (500). The Rumble though can not really create the same warm thump as the RB. So, in my opinion, it’s all about learning to use each one of them in the right situation. The Rumble gives you more headroom and volume, the RB is more true to the vintage idiom. But I wouldn’t say the RB sounds boomy, it’s just that with the 15’ speaker you really don’t need to push the bass knob too much. I actually keep it at 10.30 max.
Also because if it’s warm smooth character it may have problems cutting through the famous guitar wall. But just for sound and in the right environment it’s perfect.
The Rumble 500 (the one I have experience of) is a very reliable, portable, all purpose combo, not too modern, nor too vintage’y, with sufficient power for most situations.