I just spent some time today comparing the overdrive on the new Ampeg vs the Rumble. I was using guitar as a point of comparison, because I'm looking to see what amp can do both, but if it's useful to you here you go. I think on bass it would be more or less applicable, as far as the quality is concerned.
The Scrambler on the Ampeg is good because of the blend knob. You can crank the drive and not get overly compressed because you can blend in the clean signal. I found the tone pleasing. Of course on guitar I am like many, many other players who prefer tube overdrive. However, some pedals can come close. I feel that way with the Ampeg--I could proudly use it for overdrive tones. What does it sound like? The Lemon Song. Mids. Honky mids. Again, it's not versatile, in my opinion. I love that sort of tone, so it works for me just fine.
The overdrive on the Rumble stumped me at first, I admit. I was turning up the drive and then using the level as a volume control...too compressed, and not that great sounding. Finally I realized that it works just like a gain and master volume setup. So if you crank the level and then adjust the drive knob to taste that's where it's at for me. I messed around with the knobs and arrived at a setting that gets killer Zeppelin I album tones--think Good Times Bad Times. So it's not exactly a Supro everybody, sure, but it does that raw Fender breakup just fine. You want to cover Won't Get Fooled Again? Done. Again, it's not a tube tweed Fender, but it sounds damn good. I had two employees come over because they couldn't figure out how I was getting such good stuff out of this bass amp with my tele. An employee in another store had said "well, I don't know why I'm surprised--sounds just like a Bassman after all."
If the Ampeg had the drive, blend, plus a level like the Rumble does, it would be a better design, I think.