The analog levels of the Fireface tend to be on the conservative side, such that 0 on the mixer is somewhat less than what you'd expect for a standard +4 dBm or - 10 dBv level.  If you are seeing a level discrepancy, you should adjust the TotalMix faders until you are seeing the level you wish.  Since you are doing an analog transfer using the loopback method, you aren't getting a bit-for-bit exact transfer anyway, so you are not really going to lose anything by adjusting levels in this way.

You might try doing a loopback using the SPDIF connection, to do a digital transfer that would be both bit accurate, and also would avoid the analog level differences introduced by the Fireface' analog ins and outs.

As far as your piano, yes, that is a different issue, having more to do with the level the piano puts out, and the input setting (in Settings) you have for the Fireface.  Again, the Fireface input levels being on the conservative side, you might have to adjust the incoming level using TotalMix to something above "0", to get the working level you require. 

On the other hand, pushing levels to the max in a multi-track project makes mixing more difficult later on, as you'll find that your faders are all going to be down where their resolution isn't the most fine.  I don't use Logic, but if you are hitting an average RMS level coming in of around -18 to -20 dB FS (does Logic have a meter that displays both Peak and RMS values?), and are recording 24 bit, then you should be just fine.  Not every signal in a multi-track project should be touching a Peak value of 0 dB FS, in fact, you'll find your tracks sound better if you're not pounding your output bus limiter/compressor during mixdown, with all your track faders down around -15 to -25 or so.  Better to track at lower levels to preserve some headroom and dynamic range when you're mixing.