Asymmetric pre outs - sound quality issues?

It should not. The impedance and output that goes to the subwoofer is minimal, and electrically, there should be no imbalance. That said, where you put your sub and how you integrate it with your system with regards to gain, phase, crossover, and DSP is critical, or else it could cause the system to sound unbalanced. 

No problem!

Mike

You really want both L and R bass signals to reach the sub, but a simple Y connector could end up summing all of the preamp’s output depending on whether the outputs are buffered separately or not.  My guess is they are not buffered so you'll need something to sum the output without shorting the outputs together.

They make sub summing cables that have enough built in impedance to keep that from happening.

Otherwise, blisshifi is correct. One solution to all of this is to use something like a miniDSP to both equalize and sum the bass from your pre.