Doing this in the digital domain comes with a heavy price in noise and resolution. You are essentially operating the SL1 at a miniscule level and then amplifying it back up in the phono input.
Typical phono stages have about 40dB of gain, so when you 'turned down' the SL1 by this amount, it would come right off the top of the stated 96dB of the SL1. Granted a noisy club won't have an acoustic S/N ratio of more than 10-20dB so this may not be a big deal in that application.
However you will essentially turn your 16-bit resolution output into a 9 or 10 bit output (may be off a bit or two here depending on how you figure it). This is a significant cut in the resolution!
Even worse this is only in the midrange (1KHz). The SL1 will also have to digitally apply an RIAA pre-emphasis curve, which is attenuated progessively at higher frequencies to compensate for the equalization curve in every phono input. (also inversely added at lower frequencies but this isn't where the resolution drop will affect you)
At 10KHz, the SL1 has to come down an additional 14dB. This reduces your high frequency performance to a 7-bit DAC operating with a S/N ratio of only 42dB. This bit drop is clearly detrimental, even to non-technical listeners(won't speculate the drunk factor here)
The right way to do this is using analog attenuation after the DAC. Since Rane can't do do this until they come out with a new hardware platform, you need to get an outboard component. There is a thread discussing this here:
scratchlive.net
This will preserve the full resolution of the SL1 and have a negligle effect on the noise floor.
Sorry for the tech spew here... hope this shows why this isn't the best idea.
