You mention that you don't want volume adjustment, but I think you actually need it to get the octatrack on the same level as your modular sounds. For example, if you would use the Octatrack sound as a modulation source without boosting, you would only get a fraction of modulation out of it. Every +3dB is double the volume, so +18dB is a lot.
The one Farkas mentions has much more ins and outs, and I think that's what you want. But here's the one I have: Mutable Instruments Ears, and I like that one. There is only one 'in' on it, but it has an Envelope Follower and pretty heavy extra gain if you want it, the overdrive is very tasty. So the mangling can already start at the input module. And of course you can still use that overdrive when you don't use external gear.