Oooh! Nice! I'm glad it made an improvement. Good call rollo!
You don't need Dan Wright for this. I googled the input impedance is 20kOhms.
Looking at your sweeps I think the starting freq for the high pass filter should be 200Hz, but you'll want to swap caps to find the best value.
Use this calculator to find the starting cap value:
http://www.diyaudioandvideo.com/Calculator/XOver/Impedances are 20000 (the input impedance of your amp)
200Hz gives .04uF for the high pass cap
Ignore the low pass info
Schematic diagram for first order high pass filter:
http://www.t-linespeakers.org/tech/filters/passiveHLxo.htmlYour amplifier is R2. Just put the cap in series with the line signal. Maybe you can mod the RYA adapters with these caps, just let the caps hang out, you don't need the adapter covers. If you solder plain wires into the adapter )or cut out the old cap, then you can just clip lead the new caps onto the old leads for easy swap and experiment. You'l want to make it fast to switch freqs when deciding what value.
This cap will make the freq ~175Hz
LinkBuy pairs of .01uF, 022uF and .047uF. That will give you a variety of possibilities to try different frequencies by paralleling them together. Paralleling adds capacitor value. Increasing cap value reduces the freq. If you want to try very low freqs like 50Hz then also buy a pair of .1uF. Buy the lowest % tolerance available, but don't sweat it.
LinkThey offer 1st class mail for cheaper shipping, as does digikey
These caps sound very good, I use them in all my line level stuff. But if you want to upgrade to VCaps/Duelund/Jupiter/ClarityMR later you'll know what value works.