Has your 38 been calibrated? What kind of tape are you using? Do you have the user manuals for the mixer and machine? Are you aware that once a 38 is calibrated, ALL work is to be done in sync mode?
I've had a M-2600-32 since the early 1990's and like everything about it except the mic pres. To get around this, outboard units are exclusively used.
I seriously doubt that the 2400's front end will perform significantly better than what you already have. Unless you need the extra features of...
1s place all function switches in “record.”
Then engage the switch that actually says “input.”
Then apply your signal.
Chances are you will need the input level on the 40-4to be set @ 7 or 8 to match “0" coming from your Model 3.
I was always told that it was high capacitance that caused hi frequency loss. The original TASCAM LO-CAP cable made a significant improvement over typical hi-fi shop/Radio Shack interconnects. HOSA was Fostex's answer to Tascam's cable and proved to be a worthy competitor.
I've used HOSA cables and have had no problems with them. They pass signal without high frequency loss and are very robust. FYI - I think you've underestimated the number of cables you'll need. And do yourself a favor by laying your 38 on it's back. The capstan bearing will last longer and...
Bad tape with sticky shed syndrome most likely. Toss it. Clean you entire tape path and use newly made tape. Forget NOS stuff. It's a false economy that could possibly ruin your machine.
It's not just the "shed" that does damage. Sticky-shed tapes are extremely abrasive, so much so that heads can be ruined. Motors aren't too happy trying to pull that stuff either.
I would never hang any musical instrument on a studio wall because their sympathetic resonances would be problematic. IMHO, the areas to address are the floor, the creases where surfaces meet, and the window.
Original Great River MP-4 with direct outs, original Spectra Sonics 101 w/502 eq's, Deane Jensen Designed dual-opamp, Great River MP 500NV. All are squeaky clean except for the NV.