Understanding ohms

Old gear is really hard to quantify today. Often using high levels of negative feedback to get low distortion numbers, the sound of these older receivers is not always that great. Personally I would not run vintage equipment very hard, I have done that in the past and blown up irreplaceable parts. ML Montis can be a difficult load, use caution.  

If your Pioneer SX1980 has pre out jacks buy a good power amp and use that instead of the receiver's amp. A Crown XLS 1502 is a good recommendation (~$400).

Really depends on a few things.  In terms of ideal, yeah, sure, an ideal amplifier with no limits on current would be ideal, but dropping impedance in the right places may drop output and be euphonic.

In the case of ESL's though, they tend to drop in the top end, so it may mean you have a lot less treble than you could have otherwise.  Worth listening to alternatives.