What cables can lower my forward treble.

If moving around interconnects doesn’t work for you don’t forget about power cables. I use 2 different power cables on my outboard power supply going into my Lumin streamer depending on which signal tubes I’m using in the preamp. With the Telefunken type it’s to bright with main power cable so I switch to a more shielded power cables which helps relax things. With Mullards,RCA’s and others I use a more transparent power cable. 

If you want to try a very affordable set of speaker cables that may be a benefit for the issues that you're describing, try a set of 14 AWG Micca speaker cables from Amazon for under $50.  I tried them because they got great reviews and at least one person preferred them to their Nordost speaker cables.  I didn't like them, but it was because they rolled off the highs a bit in my system.  If 2 meters  is long enough you can try a pair for about $25.  It would be a data point to potentially give you an idea for how much of a difference speaker cables could make.

knorpi - Have you made a cable choice?

Tom

lower cardas (twinlink or hexlink)

belden copper 8402

lower model older straightwires or wireworlds

@treitz3 

"knorpi - Have you made a cable choice?"

I don't think he's been back since the he started the thread.