While I can't speak to the quality of the pres on the MR8Hd, I do know the MR8 has a rep for poor quality pres and a lot of people on this forum in the past would use outboard pres to boost the signal.
The VF80 has the same problem. I use an Alesis MultiMix for pres which I find to work acceptably. Others have used the Behringer UB802. Since the MR8HD has four pres, you might need to get two mixers if you plan to use the maximum number of tracks one can record simultaneously.