I just installed a new Bluetooth head unit in my '75 last weekend. While I was in there under the dash sweating away, I removed the foam block to give me more room to work. When I was done, I decided to leave the foam block out - what really is the purpose of having it in there? I guess giving the passenger someplace to rest their feet, like a dead pedal. But, really, given the seating position in the 914, normally your legs and feet are just going straight out ahead of you anyway, so I actually like the additional space you gain from leaving the foam block out.
That being said, it would be great to have a subwoofer in the car. The head unit I placed in my car was a cheapie model. Given that it only has two small speakers in the footwell, even a high-end head unit and amp probably aren't going to give you that great of sound. But, having that space where the foam block sits would at least give you the ability to install a subwoofer and help a little.
I would definitely be a buyer for a reasonably priced, custom made subwoofer enclosure to fit this space. I would call "reasonably priced" to be $150 on the top end. I think I paid $130 for a custom subwoofer enclosure for our 944 Turbo for the right rear wheelwell back in 2011.
One other thing, even though it is covered by carpet and not meant to be seen, that foam block may be the ugliest, nastiest, most amateurish looking part I have ever seen come out of a factory. When I pulled back the carpet, my first thought was that it was a petrified animal carcass from something that had crawled up in there and died.