I have a pair of A/D/S L-620 and a set of JBL L-36s, one of their best balanced speakers. I didn't think the 2-way 620s would stand up to the latter, but they are one of the best 2-ways I've heard -- not as in your face as the L-36, but very clean and tight.
Interesting spec differences between the 620 and 710, too.:
Freq response 620 = 30--20kHz, 710 = 40--23kHz
Tweeter: 620 = 1-inch, 710 = 3/4-inch
SPL: 620 =92 db @ 1 watt, 1 meter, 710 = 93 db @ 1 watt, 1 meter (basically the same)
Recommended minimum watts from amp for power = 15 for both
If it were me, I'd get the 4311B, particularly if they are local and no shipping -- that is one fine JBL monitor. I don't think the L-710s would be that much of a step up from your 620s (although I have not done a direct comparo). You will certainly notice a difference between the 620 and the 4311B, though!
Let your ears decide by listening to both.