Hey Rob,
I still do this when conditions are right. Having the least amount of electronics in the signal path results in the fastest, cleanest, and most dynamic sound. Not everybody's cup of tea but I generally prefer it.
When there are self-powered monitors available (which are basically speakers with built-in power amps) I attempt to connect this way. It can create ground loop issues if you are also sending a signal to the PA so sometimes you need isolation transformers (DI boxes) to avoid hum. Also, since there is no EQ in line other than what can be done on the instrument, results will vary. If the monitor doesn't sound nice flat I will surrender and ask to have bass only sent to my wedge in the conventional way from the mixing board - so it can be EQed. (The wedge will have it's own EQ so this can be done without changing the tone in the PA.)
The 1/4 outputs of our series electronics are unbalanced and often so are the inputs of power amps. If the inputs are balanced you may need an adaptor.
Any shielded cable should do. At line level the fidelity of the cable is less of an issue. (Series electronics are less effected by cable length, capacitance, etc... than passive instruments.)
The last thing is, our instruments can get close to professional line level when the outputs are cranked but they usually aren't quite as hot as a dedicated preamp. So depending on the power amp and speakers you try this with, you may miss the additional gain of an inline pre (and EQ). If you like the flat, super-clean tone you could add a very clean boost amp before the power amp to get the full volume potential.
Keep experimenting!
Jimmy J
