I have always stood by "try before you buy." However, my little brother believes he doesn't need to try out a keyboard amp before he buys it, because the EQ is much flatter.
His claim is that the sound won't differ much between keyboard amps, and that the only concerns are durability/wattage, which he can discern from reviews.
Is this true? Why or why not?
Your little brother seems to believe that the EQ is the only part of the amp that can color the sound. Sure, keyboard amps aim to reproduce the sound of the instrument as faithfully as possible, within their design constraints. But that's just as true of home stereos---does he believe all home stereos sound the same?
The preamp, the power amp, and (especially) the speakers all color the sound to some degree. It's extremely difficult and expensive to reproduce sound faithfully, and no two amps will do the same job. If he doesn't feel like trying out keyboard amps before he buys one, of course that's his business, but to suggest that all keyboard amps basically sound the same is ridiculous.
While there may be an element of truth in this (certainly you hope for a flatter EQ in amps designed for keyboards/synths as you really want to define your tone using the instrument) there is enough variety I would really recommend trying out a few amps first.
All types of amplifier have slightly different responses over frequency / power and you may find one sounds much better to you than another, or that one is better for a particular tone but not for a different tone.
To my mind there is never a substitute for listening with your own ears. At the end of the day, all reviews are opinionated and I bet your brother doesn't just read one review - he reads loads to get an overall balanced opinion. I imagine he also has favorite reviewers whose opinions generally coincide with his own.
That said, in this modern age of buying online, listening beforehand is not always possible. For me it depends on the value of the kit and how important it is to me. If I just need a cheapo monitor and any one of 100 will do then I check the reviews and maybe buy online without listening. If it's something I'm planning on using for the next 5/10 years and I've just got a second mortgage to buy it, damn right I'm gonna have a listen first!!!
To add a slightly dissenting opinion: I would be OK with going on reviews for a keyboard amp, if it is cheap. For anything that is a significant investment, it's worth listening first.
Regardless, I would say you need to listen to the amp, because even for equivalent specs, the acoustics characteristics of the amp play an important role in the final color of the sound, the presence of the sound you get, the point at which the amp starts clipping sounds... Sure enough, you can tweak EQ, but in the end, not every amp is created equal.
