Do you hear the brightness with all your sources equally? There could be a potential impedance mismatch. ARC suggests that the minimum load the LS-25 sees is 20 Kohms, abd the X250's input impedance is 22 Kohms. So you are threading the needle. Do the X250s have single-ended inputs or only balanced (XLR) inputs? I ask because the output impedance of the LS-25 is higher via balanced vs. single-ended connections, so it could have less control over the X250 via balanced connections. My guess is the LS-25 has a highish output impedance in the bass region and you are getting attenuated bass as a result of the mismatch and so the sonic picture sounds tipped up and bright. If all your sources sound equally bright, my bet is that is what's going on and you would be better off switching back to an ARC amp or if you want to stick with a SS amp, you should get one with a minimum input impedance of 50/60 Kohms. Of course the best way to do it is to audition whatever amp you get to replace your X250. The other option is to replace your LS-25 with perhaps a PASS pre amp that has a very low output impedance and keep the X250.
GGood luck.
GGood luck.