I use my IOD fairly regularly - either in rehearsal through a QSC K10 powered speaker, or through my SVT-3PRO power section and a Euphonic Audio VL210. I'm playing an Epiphone Jack Casady Signature through it (all Casady, all the time!).
I think William Blake said something to the effect that "you never know how much is enough until you know how much is too much." I turn up the drive until it's too much, then back off a bit. I also use a little bit of clean boost in front of the IOD to push the first tube preamp stage a little harder, your active Spector may be hot enough to do that.
As mentioned upthread, there's a tendency for the sound to get dirtier as the power amp warms up. I find this works out really well on a gig.
And the bias adjustment makes a difference. I took some measurements and did some calculations, and unless you get into really extreme hot bias settings, you're probably not going to hurt anything by twiddling it. As the bias gets hotter and the plate current rises, the B+ voltage sags and keeps the plate dissipation reasonable. I actually set mine by measuring the B+ and setting it to match what the schematic shows, and that seemed to be the sweet spot.
If you use an external speaker to load the power amp, be aware that speaker can act as a microphone - it can potentially feed back if the stage volume is high enough.
Someday maybe I'll build a reactive speaker load.
