I'm not sure a comparison with my experience will be all that valuable, but I may be able to offer some hints and guesses.
I've never had a problem with a limited contrabass range on a tuba (BBb or Eb). On either of my current tubas (see my sig), I can play down to the fundamental and below. I have to think that your lower contrabass range was a matter of practice. Most community band (BBb) tuba players I play with have a range from the F an octave below the staff (or maybe just a little lower) to maybe the G at the top of the staff. They typically accomplish this by using large mouthpieces (actually too large for them, generally) and just giving up the high range. The director in the band I used to play in (a tuba player himself, with an advanced degree in low brass, and a university faculty member) always used to ask me if the G and above were "too high". Of course not. I like playing the Eb because it sounds better in the range above there, but actually producing those pitches shouldn't be a problem (even on a BBb horn). But what often happens is that people adapt to the range that's required of them -- and in a standard community band, the tuba parts just aren't generally very demanding in terms of high range; and they aren't very demanding in terms of low contrabass range. So people's ranges tend to converge on what's required. At least that's my view.
I think you're still playing with that range in your mind
, and you just need to work on your embouchure, your range, and HEARING what you want to play.
Unfortunately, both you and I now are at the point where muscle tone everywhere is flying out the window, and so embouchure development may require additional work -- but I'd recommend just a little bit a day in terms of extending the range slowly.
Also, don't be afraid to cheat -- at least initially. When I play my euph just for kicks, on fairly easy music, or as a "tenor tuba", I use my DE mouthpiece (which is fairly large by euphonium standards). When I need to play in the "real euphonium" higher range above the staff, I cheat and use my Wick 5L. After I've done that for a while, I then discover that I can switch to the DE and have pretty much the same range instantly. But honestly, given circumstances, I feel no shame in using the smaller mouthpiece. In fact, most of the euph players I encounter in community band are probably using ones that small or smaller. 
Just one other comment: I simply don't like the Wick SM mouthpieces. At least try some of the others. There are places you can order them on trial and then return and have it cost you only the shipping.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)