When it comes down to looking to purchase expensive equipment that is intended to work together such as amp and speakers, I really do feel there is no quick resolution (really need to do a careful audition otherwise it is a gamble to spend that amount when you may not gain anything beyond a well built-specc'd mid priced item).
If interested in purchasing an expensive amp, it needs to provide some benefit you cannot gain from a moderate priced product.
The only way to be sure of what you gain, is to be aware of the speakers characteristics and how this sounds combined with an amp.
What compounds this though is that you will require certain songs to highlight the aspects that may niggle with you, and realising which songs work to show the characteristics can take a bit of time.
Its difficult to suggest what music to use as each person has different tastes, but considerations should include album/songs that have:
1. Not just very good but also average sound quality recording
2. Masking-hissing that can be heard when vocals sung, or a sampling of instruments or other track.
3. Acoustic recording that can combine piano-guitar-voice and using the middle and upper scale, while the piano and guitar played hard-forcefully.
4. Album that can sound average recording on less spec amps but excellent on a good amp. This is really subjective but personally in my opinion I have noticed this with some albums.
5. Very fast rythmic music with timing that is easy to follow but a lot of it going on.
I guess traits your looking out for beyond the vague subjective are characteristics such as break-up, too much hiss,etc.
While the link I am providing is more about subjective evaluation of speakers, quite a few of the rankings can also be applied to amps, when using the same speaker but with different amp combinations.http://www.linkwitzlab.com/Loudspeaker%20evaluation.htm
I am stating this from my own experience, the difference is that I already had the speakers and knew them very well, which allowed me to select specific album and songs to listen to various amps in conjunction with them.
My own criteria ended up having to decide between complete sound stability (no break-up or hiss becoming too annoying) combined with tonal balance/clarity-articulation-speed, and another that had more excitement with regards to dynamics/sound stage but left some albums and songs either unplayable or just feeling like it was an average recording.
Remember it is not just the positives you are looking for, but also negatives that are niggles and quirks that can cause you to not feel fully satisfied, possibly resulting in the upgrade itch that quite a few feel after several months to a year.