franc, sorry, I heard the BETA 20, but not in an A/B listening which makes it pretty much a moot comparison ... I have not heard the Rti6, only the Rti10 again not in an A/B comparison. the B2 was not and IS not in my radar because I simply do not like how it looks (with or without the grill).
I have the BETA 50 as well, of course if you were to A/B the 10's and 50's bass would naturally be stronger with the 50's (and the 20's). but with a properly calibrated subwoofer, both should sound "similar" enough. another thing to note as well is why the BETA 20 will have better bass is that it's enclosure is HUGE! ...aside from the bigger driver.