One of the things I've learned from the guys here at BB is patience. an M70 will turn up, it may not be the price you want, and yet sometimes it might be almost perfect...... but you just don't have the money right now.
The M70 is a special box and is much lauded here. After all it is a Grail.
Comparison of bass between the 656 and M70? There is no comparison. Chances are when you capture an M70, you won't sell it - ever. There are guys here that have two and more.
Enjoy the 656. It is a great sounding box.
Cheers
Brownlow