the first time – need advice about amp for magnepan 3.7's

[Feanor]

Mike, the classes of amplifier available today are 'A', 'A/B', and 'D', (though there are other types). You can read up on amplifier classes HERE.

The Pass Labs amps in the X series are class AB; their XA series amps are class A. The Xnnn.5 amps, including the X150.5 and X250.5, are "high-bias" class A/B meaning that they extend class A operation to relatively higher power levels.

To address you question about "balanced" outputs, there are a caveates but in general it's better to used balanced connections between components if balance outputs and corresponding balanced inputs are available. The Pass Labs amps have balanced inputs and it would be preferable to use them with your LS17.

Here's a nice pic of my X150.5 ... ...