Replacing/fixing the cable in an Ernie Ball- Invalid Link Removed I've done it a couple of times for lazy guitarists in the past with some difficulty (it isn't something you want to try to do right before or during a gig), but good luck fixing most other mechanical designs that easily. One benefit of the Morley design I guess is the easily replaced light bulb (although if your problem in a Morley ISN'T the light bulb, then you are out of luck).
As far as the tone "raping" as many guitarists put it- a passive volume pedal is essentially like putting an added passive volume control in your bass- how much it affects your tone depends on the tone you are going for and your particular setup. The reason the tuner pedal out can be an issue is because if the tuner is on, its circuitry will load down the thru signal (more or less depending on the tuner). The issue is that many probably think that putting the tuner through a volume pedal takes it out of the signal path, but in reality it is still loading the signal just like running it in line- turn the tuner pedal off when not in use and it shouldn't be an issue. All of this stuff will likely be far more noticeable for guitar players, because the mass of their "tone" is in the highs and high-mids most affected by impedance changes and loading. If you spend alot of time sculpting a brighter tone on your bass this kind of change can also be very noticeable.