The Sub37 is an incredible synth, but it's not 100% the best for integration with a modular, mainly because of (a) it's form factor (it's big and it will be hard to fit it next to your modular); (b) its lack of CV out's; (c) its limited CV in's. It's neither modular nor semi-modular, so while it works great as a stand-alone synth, you can't really integrate most of its functions with the modular in a fully satisfactory way. Of course, you can use your Sub37 with your modular and I'm sure some people do--it's just not ideally suited for that use.
(Note: I have a Sub37, a Little Phatty, two Moog Mother 32s, and a growing modular.)
It really depends on what kind of music/noise/exploration you're trying to make and what you want to do. What would you use the Sub37 to do? That's the key question in terms of whether / how to integrate it.
You could get a Midi to CV module and use the Sub37 as a controller, but it's a pretty big controller and you may not be using much of the functionality of the Sub37. Are you going to have your modular in a case in front of you?
Personally, I find it easier to leave my Sub37 to the side (it's in a completely other part of my studio) and use it for other things. The Mother 32's on the other hand are perfectly designed as a way to get into Eurorack. They easily integrate into a modular set-up and are great value for money. They are small and can be put into a case with your other modules, or can sit to the side. To get a similar number of modules (sequencer, keypad, LFO, VCO, VCAs, mults) would cost a lot more. They can also work as a great extra oscillator for your Sub37 and have a MIDI-in.
So, I would also recommend that you consider getting a Mother 32 and seeing how you like it. You may end up selling the Sub37 or you may end up keeping it -- but probably for non-modular use, I would expect.