Reserves of power in an amp can improve transient response .. clarity
Key word: "can". If those power reserves improve transient response(why does it have to be only the transients that require power?), then that power was *needed* and *used*. It wasn't "extra".
The rest of the world refers to unused power (or signal level) as "headroom". It's a buffer zone, but it's also wasteful to have capacity and never use it.
And widens the range of speakers that the amp can control ...
I'm up for a PM discussion about power and loads ..
For the same power: if a speaker is higher impedance, you need more voltage but less current. Lower impedance, less voltage, more current. The hardest things for amps are peak current and peak power, but those needs are determined by speaker reactance at particular frequencies, not nominal or even lowest impedance. However, an amplifier's capabilities in those areas aren't included in the single "FTC" number that amps are sold by. Just buying an amp with more nominal power isn't a guarantee that you can play as loud as you want, if you have exceptionally difficult speakers.
It makes more sense to spend the money on more efficient and easier-to-drive speakers that will work with more amps.
I was trying to help the OP ..
My concern is that you're advising that he obtain an amp larger than he needs, when he's specified a budget and is looking to upgrade his speakers as well. The more sensitive his new speakers and the lower his preferred listening level, the less power he'll need. Right now we don't know either of those variables.
In fact, it may be worth his money to refurbish his existing receiver. A refurbished used equivalent would probably cost more. If he upgrades to a modern receiver, within his budget he'll end up with around the same power. 120W per channel is plenty for most speakers at volume levels below the threshold of hearing damage, at typical listening distances in typical rooms. Sure, there are exceptions, but right now you're doing the OP a disservice to advocate "get the biggest amp you can find", especially while making the false claim that it will always make everything sound better.