So I was recording a korg sv1 today and sent the stereo trs outputs to my pres. I sent the left side to an Avalon 737 and the right side to a focusrite 428. However I got a huge amount of hiss in around the 10k region when doing this. Does anyone know of what this might be? And do you think it could be that I used two mono tracks with different preamps?
Are you running the Korg Master Volume up at least half way (control the record level with the preamplifier gain controls) and connecting the outputs into the preamp's line inputs (not "mic, and not "Inst.")? Keep the Korg output high and the preamp line input gain low. Control the recording level using the preamp gain controls.
Be sure you're using the "line" inputs on the preamplifiers, not the "Instrument" inputs. On the Focusrite that means the back-panel TRS jacks only, not the ones on the front panel. On a VT-737 the line input is only on a XLR, the front-panel TS jack is an Instrument input.
The SV1 outputs are actually unbalanced (TS) or balanced (XLR)s. The signal is lower on the TS outs. They should work with the TRS inputs if you use TS to TS cables, although the output is low. Ideally, to properly connect to the balanced inputs of the Focusrite 428 , you should be using a balanced XLR to TRS cable, the XLR outputs are higher level. For a Avalon VT-737, you need an XLR/XLR cable going to the rear-panel "LINE" input. Do not use balanced XLR/XLR cables for line-level input signals to either of those pres' "Mic" inputs.
Using two separate preamps and two mono tracks won't make any difference as far as HF noise goes.