I used to use a boost when recording loops to keep the volume equivalent when necessary (depended on the other processing I was using at the time). I actually used to use both delays on my board, had a DL4 used almost exclusively for looping and a DD-20 for delays (guitarist in my cover band does the same) but when I went to the M9 and was able to do loops and tap tempo delay at the same time I sold the DL4 and DD20 for the convenience of one pedal. And to cover expenses.
Just throwing this out there, but the M9 is about the same size as the DL4, has a longer loop, and better tone/volume controls for the loops IMO, but that all comes with a price (and a boat load of other effects

). That said, I haven't been doing as much live looping now as I used to (that band changed membership/direction) and I rarely use it for my other bands so it's really just stuff I'm working on at home. That said, playing with the M9 at home I haven't had any trouble with volume issues on the loops.