The Interbass is a Swiss Army Knife: it combines in a small package features that are usually not present in small packages (either combined in bigger packages, or in separate packages).
What makes the Quilter Interbass unique AFAIK is the combination of preamp features and small amp in a tiny pedal shape package.
As soon as you remove one of these elements, you get an almost infinite list of possible alternatives.
Examples of alternatives if you remove one of its charasteristics:
- Same features with no size constraint: any amp
- Same size and features execept the you don't need the power section, just headphone out and line out (for example you can plug it in a combo): dozens of preamp pedals
- Same size, with power amp but more basic features: micro heads like the Trace Elliot ELF Basshead.
So basically if you want an alternative, you have to abandomn either the small size or the ability to plug the preamp directly to a cab, or some specific features.
By the way, the Interbass doesn't have a real DI out, it's a TRS line out, you have to use a TRS to XLR adaptor if you want to send a DI signal to the FOH.
Let's focus on the Trace Elliot ELF Basshead because it's very similar: compared the the Interbass, I believe it only lacks the FX loop and cab sim. And the controls are on the front instead of behing on the top. If you don't need FX loop or cab sim, then the Elf is a possible alternative.
In terms of tone, I believe any modern amp can let you get a mid forward tone like Bruce Thomas or Cake (I love them too by the way).
By comparison, I don't care for the Elf because of its ugly color, and more importantly I think the top facing knobs of the Quilter are much more usable that the front facing controls of the Elf.
The Elf has more power (200W compared to the 45W of the Interbass), but:
1. 45W is louder than you think, it's more than enough for home practice, and probably enough for small reheasals
2. When I need more than 45W I don't need an amp to be so tiny
I'm also debating buying the Interbass - I don't really need it because I've already have several amps, but I've read several Talbass comments that say it sounds incredibly good, specifically with a great tube feel when you push the gain. When the the Interbass came out, I thought it had the same tone (preamp section) as the Quilter Bass Block 800 that I already have and love, so I didn't buy it. But I saw that some TB members use it as an external preamp for the BB800 because of that cool tube emulation associated to the gain knob, that the BB800 lacks.
So I would prefer the Interbass over the Elf even if I don't need the cab sim and the FX loop.
Last, if you don't really need the 45W because you will meanly use it with headphones of the line out, then you have an incredible number of alternatives, so many preamps available for bass guitar nowadays, you will enter a huge rabbit whole.
Sorry for the very long answer - I didn't want to write such a novel initially, but I found out that your question was very interesting actually.