After reading reviews here and elsewhere I initially chose the BC20. It sounded very good, but was missing a little something. You really do need a 10" speaker like in the BC40 to produce a full bass sound.
The clean tone at any volume is ideal for practicing as every subtle flaw is clearly audible. It makes you better faster as you hear inconsistencies in playing technique. You hear the instrument, and unlike the Fender Rumbles 15 and 75 --which also have a (nice) specific sound and have their place-- all frequencies come through clearly and at equal volumes with Warwick. The BC20 also did this, but was missing some bottom end, especially compared to a Rumble 75 I was also using at the time. So after a few weeks I sent it back, and with great service from Thomann upgraded to the BC40. The improvement was instantaneously noticeable and months later I am still very pleased and highly recommend the Warwick BC40.
Fender MIA J-Bass; finger style; rock, blues.