+1 on the Sansamps. Any of the pedals are good but you can also get a Classic new for under $300.

EHX has some tube pre-amps such as the English Muffin that are worth looking at. So does Carl Martin.

What about a modeler? You can get a Line6 Pod in your range.

I personally use a Roger Linn Adrenalinn. You can get one used easily for under $300.

It’s a real PITA to program but it’s insanely deep. You can use the sequenced filters and effects for some serious mangling but I also just happen to like the amp models. I prefer it over my old Pod because it has some really great bass amp models that I thought are more satisfying.

I’ve never used an Octatrack but I can imagine that you can find other uses for an Adrenalinn that play to the Octatrack’s strengths. For example, patching the Adrenalinn’s drum sounds into the sequenced effects have given me some truly out there rhythmic squelches that are Machinedrum-esque.