Check this design please, a bit low SPL is this normal?

Basically the looks of this sub I want to build are very important to me and so this is the best compromise of style and size to response, but when I looked at the dB SPL plot it made me a little worried as it was a bit low compared to my current monitors (Monacor SP-210) and so would appreciate a little input! I don't think it's too bad is it?