speaker or amp distorting

i have jm reynaud arpeggione floorstanders that are a small two way with soft dome tweeter and 6.5 inch paper cone in a transmission line design. they are reasonabley efficient 90db. my amp is a modified jolida 102b with el84 tubes and 25 watts of power. on simple music with without too many dynamic peaks and modest instrumentation i.e. four or five instruments or less this setup is magical. however, with big band or vocals when things ramp up dynamically the treble seems to harden and hurt my ears. occasionally i will hear some distortion. is this the amp or the speaker or something else? thanks
Probably the amp. I don't see how they get 25 watts out of EL84's. You'd be lucky to get 18. If the stock tubes are still in, that's another source of poor sound. Tube rolling may help a little but a bigger amp will help more.
Amp, sorry to say. When it goes from magical to painful, it's clipping in the amp, almost always.

Can you borrow an amp with 100 watt or more? (90dB sensitivity isn't exactly inefficient, but it's going to push your 25 watt amp to it's limits, particularly if your room is fairly large and has a lot of plush furnishings).

My speakers are rated at 91dB sensitivity and they didn't really start to sing until I got up around 200 wpc. I have lots of big band recording, big orchestra and jazz with powerful bass. I can't imagine playing that around 85dB average SPL with only 25-watts.

One question, I'm assuming that you're keeping the input level fairly constant and your source is the same for small groups and big band. If that's true, then I'm still betting on the amp. If it changed with source, then the input level might be too hot from one of your sources, but I'll bet you're using one source at around the same level for all types of music.

I lied, one more question, is this happening with a digital source or a turntable? If TT then you could have cartridge mistracking from too little downforce, or just a underachieving cartridge.

Probably amp but your speakers can't take a lot more than 100 watts anyway.

A small two way is not ideal for big band. For example yours has only a 1.5" voice coil for the woofer....which is only a half inch bigger than most tweeters - typical of small two ways but as you can imagine it will get hot very quickly at modest levels and the heat will cause thermal compression making it sound dull when approaching realistic big band levels. This is quite normal for small two ways - so don't push it - i don't think a bigger amp will completely help in your case - so be careful spending a lot of money on a big tube amp until you are sure your speakers can take it. (Borrow an amp first)
thanks for the advice so far guys, the frustrating thing is how good this system sounds when it is not distorting. it seems i need to proceed with caution and find a way to get bigger sound without losing the musical nature of this setup. two things i know: i want to stay with tube amplification and i dont want speakers with metal dome tweeters. my musical tastes tend to be fairly diverse so i need to find a way to make my system more versatile.
Don't get hung up on tubes vs. solid state or metal tweeters vs. silk, etc., etc., etc.. Just listen and let your ears guide you. Arbitrarily going for tubes may have already led you down one dead end. You've got two choices, either get speakers that match your amp or get an amp that matches you speakers. I'd usually go for the second option.

Borrow a good amp with more power (SS or tube) see if it doesn't clean things up.

thanks for the advice so far guys, the frustrating thing is how good this system sounds when it is not distorting.

Unfortunately getting clean beautiful sound at realistic levels is exponentially challenging - particularly in the bass. It is generally quite easy to get good sound at modest levels/dynamics. Most systems commonly available will all sound pretty good at low levels (otherwise they wouldn't stand a chance to sell)