With my ACT 2.2, I have used a variety of amps. Pass XA.5's, Bedini 45/45, several SETs, push pull tube, even a Tripath based Red Wine Audio 30.2. All have worked fine.
The ACT's 500 ohm output impedance (average) is pretty low for a pure tube preamp so it should interface well with most amps.
Like Lloydelee mentioned, I view the amp/speaker as a whole. Some amps will "shake hands" with a speaker better than others. Your 350SA will be a lot happier driving far more brands of speakers than my SET would. So consider the speaker when/if you decide to change amps. Don't worry too much about the ACT 2.2 fitting in.
The ACT's 500 ohm output impedance (average) is pretty low for a pure tube preamp so it should interface well with most amps.
Like Lloydelee mentioned, I view the amp/speaker as a whole. Some amps will "shake hands" with a speaker better than others. Your 350SA will be a lot happier driving far more brands of speakers than my SET would. So consider the speaker when/if you decide to change amps. Don't worry too much about the ACT 2.2 fitting in.