??? How Do You Determine How Far You Can Take An Amp When Choosing Speakers ???

Wow, that is a challenging question. It’s a bit easier if thinking about the beginning of the audio chain, since if you have a poor signal, better components may make it sound worse. But at the speaker end, unless you are under powering the amp, generally, the better the speaker the better it will sound... I guess unless you are getting a tremendously detailed speaker... for example Magico... too revealing. I have heard Magico make some pretty good equipment sound bad. Hmm, thinking out loud... obviously. 

Most of us are trying to set ourselves up for the next upgrade. But sounds like he does not want to. So, I’d go for the 2x to 3x area. If he doesn't know he is going to upgrade in the future... then being conservative makes sense. 

I suggest something like some vintage tannoy’s with the 12 or 15 inch concentrics (monitor gold) that maybe need a bit of work. High efficiency, amazing sound especially the bass response. 

@mulveling may have some advice here if that is of interest. 
 

The other point I would make is spending what may seem disproportionate on speakers may not be a bad thing.

For example, for about 3 years my Harbeth 40.3’s did double duty as 5.1 mains driven by a $2K avr (Yamaha RX A3080) and I often used it for streaming music for casual listening.

It sounded remarkably good compared to a dedicated digital source and through a T+A 3100 HV 

So get some really nice but well priced efficient speakers that could hang for an amp upgrade or 2

In Absolute Sound there is a review of Kef Meta 11’s and they use a Peachtree and then much more expensive amps to show the difference a quality amp will bring out in the speakers. But they state the differences and while good, are not extreme. This might help you understand the limits of what your amp is capable of. Also at your price point you might consider the Meta 7’s which might be available at your local chain store. Cheers , Mike B. https://www.theabsolutesound.com/articles/kef-r11-meta-loudspeaker/  

After being done with the Radio Shack "kid stuff" (late 60s) and only having one real human source for advice.  The guy was a neighbor and an EE and had a single Klipsch Klipschhorn driven by all mono Mc gear.  Fat story slim...he advised buying the very best speakers you can and work backwards to the source (remember it was only LPs, radio and tape).  

I took his advice and a few years later bought a pair of Hartley Reference loudspeakers and I still have them (tri-amped).  Went through Sansui amps, David Hafler amps, ARC amps and landed on Pass Labs amps with a Ref6 about a year ago.  Took a while to climb that ladder, but I'm glad I did it that way. 

For you friend's son:  Look and listen for a long time and purchase the speakers you love.  They will not disappoint.

PS.  Those Hartleys cost $10K back then and everyone thought I was bonkers, crazy, irresponsible and insane.  But I'm still smiling.

Regards,

barts 

There is no rule of thumb. Once you get the BS budget etc. out of the equation it is quite simply sound appreciation and how do you give that in advice form. Certainly speakers have an operating range in specification posted. Doesn’t take a rocket scientist to realize the amp should have headroom. As said sound is personal limitations are mechanical. My Speaker system being fairly inefficient require considerable current, that said I chose a necessary by-amp style of amplification that weighs in at the far side of 200 pounds. With safety considered I love the sound..at any volume !