Hi Toril,
As you pointed out, you were already using just the PSR 2100 for a long time with no problem.
Then, when you started using the S970 you got the negative comments...
So, I would assume that, since the 2100's amp is definitely not as powerful as the S970's, you should not have any problem with it's volume.
Maybe the problems you are having may just be how the main equalizer is set.
(And possibly the difference in the angle and direction of the speakers on the S970?)
My suggestion would be to take your 2100 with you, and play the same simple style or song on BOTH keyboards to see what the difference is (especially from where the audience sits).
How does the sound carry? Is it muddled, or clear? Is there too much bass on the S970?
As far as equalizer settings go, there are posts in this S970 section about what other people have done to tweak their keyboards - I believe it has been a well talked about subject.
And Gary might be able to help you there as well...?
Surprisingly, I have not had any complaints when just using my S950, after playing my 2100 for many years...just luck I guess.
As a matter of fact, there are a couple of small venues where I now only use the S950, where in the past I would have had to amplify the 2100 a bit.