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.)