My Mac experience is with G4 so things could have changed.
I found the level control for the analog input to not be optimum. I put a symmetrical sawtooth into the analog input of the Mac while monitoring the output of the USB box (ADS RDX 150). With the gain on the Mac set to max clipping occured at 3.4 VPP at the input and 2VPP at the output. With the gain on the Mac turned down clipping occurred at 6.6 VPP input but only 260 mVPP at the output. I would have expected that for any setting of the gain the same output should be achieved and decided that the designer screwed up and rejected it. I should have tried more input levels and gain settings.
Using the ADC box and Toast 8 the gain in the Mac is fixed. Clipping occured at 2.25 VPP input and 1.04 VPP.
It appears that the Mac soundcard gave ~6 dB more headroom than the Toast gain setting for the external box but the funny in the gain control in the Mac caused me to avoid it.
Especially with the newer Mac YMMV.
melsmcintosh wrote:This is what I think I've settled on for low budget recording my LP's on my Mac g5:
Technics SL1100A with Stanton 681 EEE, (I have a ADC XLM MK III but the stylus is shot)
Cambridge 540p preamp going through my imic USB interface to by pass the Mac audio input stage.
Any thoughts good or bad?