Need some treble

solution would be either an equalizer box or preamp with tone controls. 

If there is a tape loop, put a Schiit Loki there

Second the recommendation for a Schiit EQ -- they have three options. The Loki Mini is only $150 and offers four bands. The Lokius is $300 with six bands (and offers balanced XLR in/out as well as RCA) and the top-of-the-line unit is the Loki Max for $1,500 which is six bands but with all the stops pulled out (relay potentiometers, full inductor coils, remote control, etc.)

If you are streaming and using Roon software, Roon DSP is the ultimate tone control.  No special hardware required.

Did you verify that the tweeters in your speakers are working?