He had no clue about the cap issue. Actually I think I was the first one to tell him about that. When I asked him why did he modify the Ric, his answer was "because I didn't want to sound like f***ing Chris Squire".
He didn't know about the cap issue, true, but a lot of people didn't do that in the 70ies. I only learned it here myself. Bass ooomph wasn't so much the issue, as he was using Martin cabs from a certain point onwards. What Roger didn't like was that the Ric didn't give him a clean sound, he wanted an undistorted, non-overdriven bass sound like he believed to hear on American recordings at the time. That Machine Head and Made in Japan overdriven sound that people find to die for today, that wasn't tidy enough for him.
Fender experiments had not satisfied him either. The P Bass on In Rock was too inaudible to him (he was right) as it lacked detectable mids, the Fender Mustang used on Fireball lacked ooomph in a live setting. You only heard Roger well on DP records once he used the Ric, but that sound was too overt for him too. When he returned to active playing with Rainbow in 1979 he used a TBird (which was a lot more inconspicious in the Rainbow sound than the Ric in DP had ever been, but that is what Roger wanted) until he broke its headstock off on stage. He's on record for saying that had he known how a TBird sounds while playing with Purple, it would have been his choice over the Ric.
That said, I will forever identify him with a Ric, look- and soundwise. Whenever he drags out the Ric today for Smoke On The Water in favor of the Vigier, it draws a tear to my eye. That Ric sounds nasty and dirty, but just right. A commanding sound.