OPA627SM
Whoa, Military grade opamps, those look amazing and sound even better. Although we doubt many of our readers will upgrade to these modules as they are worth more than the Forte 7.1 we still wanted to test them out and relay our findings. We found the OPA627SM module to sound very balanced, with no confusion or congestion present. The opamps offered increased separation between instruments vocals and other aspects of the sound. The combination of the Forte 7.1 and the 627 SM produced a very warm character with great detail, articulation and exceptional clarity.

OPA637SM
Yes, we will be testing out the 637SM module also. Don't they look cool though? Well we are not interested in looks though are we? - So let's give it a listen. This opamp has always been a personal favorite and it has been used in many cards and external units that accept opamp replacements.
We have the laidback Burr-Brown character but with an extended high end and improved separation and clarity. There is no confusion or congestions on complicated musical passages. The amplifier just provides a very balanced crystal clear, articulate sonic profile.

Further Modifications

For those handier with a soldering iron, you could possibly look at replacing the JRC4580 with SOIC8 version of the LM4562 or perhaps use surface mount version of the LM4972. There are also adapter modules that would allow you to add DIP-8 Sockets to the SOIC-8 PCB footprint allowing the user to experiment with different opamps for the other channels.  There are many great opamps which could be used but going into details for the units is a bit outside the scope of this review.

Disclaimer: Such modifications are not recommended for the novice and offered here for informational purposes only. The modifications can possibly damage your card and will certainly void your warranty.