i like the speaker motor models, NOT the pure resistive load models.
those are like the THD hotplate, i feel that the speaker motor models feel more natural.
make sure you match (over match) the wattage.
there are several levels of attenuators available for each wattage level..
if you're not sure what to order, call or email them.
i use a 100 watter for my 60 watt boogie. i do occasionally dime the amp, so that's why, plus, never go under the wattage.
if you dime your amp all the time, YOU WILL WEAR YOUR POWER TUBES OUT faster.
plain and simple.
when they fail while being dimed, they will do what they do the same way if you were on a big stage, at full volume.
the best way to use the attenuator, is to tame your levels somewhere in the middle of your typical playing volume, and a bit quieter for more laid back gigs.
if you want to play at bedroom volume, get a new amp, with 1-5 watts max, and possible a master volume or built in wattage knob.
for a 100 watt, if you're going to run it hard (like, dimed) i'd suggest going for the 150 watt model.
http://www.tedweber.com/atten.htm