OK, assuming it's an 802, that would still mean mic 11/12 gain of +40 dB = -24 dBu in max. This is not anywhere near line level.No that is regarding the RME 800/802/UFX line. Ports are not arranged on same channels on all RME devices.
Reading the specs, I have a hunch that the Line 1-8 inputs have only two actual hardware gain settings for +19 dBu ("Lo Gain") or +13 dBu max ("+4 dBu") respectively, with the +2 dBV ("-10 dBV") setting being provided by applying digital gain to the latter post-ADC. The block diagram would seem to support that. This would put these inputs at a substantial disadvantage when handling super low signals. Even when assuming some non-negligible source impedance, the difference still be around 30 dB, at which point the 116 dB(A) dynamic range would have shrunk to a mere 86 dB(A), and worst-case to 80 dB(A). If then your actual peak levels aren't anywhere near fullscale, you could easily end up with a SNR reminiscent of analog media.
In sum, the question remaining is why your synths are so insanely quiet. I can get like 2.5 Vrms (+10 dBu) tops out of my crappy old Yamaha keyboard's headphone out... granted, the noisy power amp in there means it barely makes it into the 80s of dB anyway, but still.