I have a DIY unit used almost exclusively at home. Every time I have tried it in a band setting, I have ended up turning it off and just controlling the output knob. At home, it does come in very handy to be able to keep the same settings while taking it down to acceptable home levels.
I use the highest level (-12db) for late night. It does have a wet blanket effect and seems to change the volume balance of the channels. But for a late night tone fix, it does the job. I tend to adjust the attenuator and output knob, one after the other, to get the right volume. The right volume depends on the time of the day. I play around all of the other increments (-3db, -6db, -9db) all the time.
As with anything in the signal path, it's going to have an effect on the signal itself. I have a very simple, passive attenuator that has no tone compensation. It simply takes away from the sound. But I can turn my amp down by -9 or -12db and be able to get that sweet power tube saturation without getting complaints from wife or neighbors. I highly recommend *any* tube amp owner to invest in one, DIY or store-bought. If the unit fails while in operation, it can have a catastrophic effect on the amp. So that is something to think about if considering DIY.