Totally agree about one of the Bassmans,favouring the 135 not because the declared extra-power (in real life the difference is not 100 vs. 135 watts) but because the particular configuration of the 135's output stage wich is ultra-linear.Also,don't forget the other rule saying "beefy iron=no saturation=dynamic headroom".The size of the output transformer speaks for itself when a clean an punchy bass comes in question.
Someone posted the word "sag"...well,the sag appears mainly when we're talking about tube rectification.Both Bassmans have SS diodes so the sag is not an issue.
The only drawback of the Bassmans could be their channel 1's preamps.In both cases,we're dealing with the "classic" Fender tonestack without mid and this implies that specific "Fenderish" sound (scooped mids) wich is highly touted by guitarists.For bass,however,it's not THE tonestack I swear for,but this can be easily solved by modifying the ch.1 tonestack to an Ampeg-style one (mainly Gemini,it's a great one and simple too).
One short word about the box to be used with a tube head:you must be sure that the impedance of the box matches with the specified impedance of the amp.DO NOT play through a Bassman 100 on a 8 Ohms (or,worse,16 Ohms) cab and use only 4 Ohms.Otherwise,it's very likely you'll destroy the output tubes due to the mismatching.
A 4x10" seems OK regarding punch and speed but if you use a 5-or a 6-string bass,the 10ths lack the bottom.In this case,you'd might try a 1x15'' with a 2x10" on top.
Bottom line:to a certain degree,I agree that a tube amp is pure gold for guitar but somewhat critical for bass.Though,a well-tweaked 100-or-more watter tube can sound unexpectedly good.
Have fun,
Le Basseur