I have the Zoom B3. Through my Mesa D800+ and Darkglass DG500 heads, it’s a great unit.
The Fender Rumble Studio 40 really stands on its own in the bass amp market, though. It’s market is the person who wants an all-in-one solution to cover a lot of ground. The amp and cab sims are VERY good, most of the effects are also very good, it has a looper, an effects loop, aux in, stereo XLR outs, onboard tuner that actually works well with the bass (including low B on a 5-string) without being jumpy/fidgety. Onboard compressors, limiters, bass-tuned OD/fuzz/distortions, octave, touch wah, etc. USB connectivity. Compatibility with 4-button footswitch to access presets or turn effects on/off, expression pedal comparability,... weighs in at 20 pounds. Tweeter that you can attenuate or turn off. All-in-one compact solution for practice, recording...even stage.
The other nice thing is that your bass will still sound like it’s supposed to: J sounds like a J, P sounds like a P, etc. in the guitar digital amp world, a common complaint is that some of those low/mid-price offerings tend to make all guitars sound very similar.
I can’t think of an actual peer when you think about the feature set.
With 15% off you can get one new for about $300. Yes - there are plenty of options, bigger and louder, at a lower price point. But - I think if you are in the market for what this amp offers, you are not really comparing it to less expensive bigger/louder/heavier amps...apples and oranges, right?
I have a fair amount of gear, and there is absolutely a place for this amp in my stable.
Quick summary:
With my active Sire VV7 (Jazz): easily dialed in slap tones. Great bottom end. Great growl. Easy to dial in a scooped mids tone (not my preference, but certainly the presence of many).
With my Stingray: pure Stingray goodness.
With my P bass: great classic tones.