I never tried the BK-7m but I sampled the BK-5 in a music store for about a half-hour. The BK-5 sounded fine to me through borrowed headphones--about the same as a GW-8, which I owned for a while. However the on-board speakers are lousy, way inferior to a Yamaha s900 or Roland E-50.

Could the better sounds from the module be due to AFTERTOUCH? If a good controller sends aftertouch messages, the BK-7m can process them, enhancing the sound of at least the Lead voices. However the built-in keyboard of the BK-5 doesn't create aftertouch. The best experiment would be to listen to an SMF in both boards, or play both with an external controller, and through headphones.

Also, I noticed that an OS update was recently released for the BK-5. Recent Rolands have been notoriously buggy, perhaps a consequence of using what seems like different programming teams for each new model. With the recent OS patches, I would now consider one of these. I would like to know what the difference is too. I believe I would miss the lack of a style composer on the BK-7m. Why Roland doesn't release this in software is beyond me! -Ted