I don't know if this applies to cello imitation (or so much), but one of the real challenges in making a tuba sound like a string bass (at least in plucking mode) is to is to mimic the "degradation" of the sound at the end of the note. This is why some tubists can really sound like a string bass and others can't. Not sure if there's anything comparable in playing cello music, but strings do have a distinctive sound (or sound "profile", or whatever).
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)