You sort of need to get to a happy medium betweeen the stereo delay and the mono signal.
What you could do is start reducing the delay spread (and keep switching to mono to compare) and find a happy medium between the two. So you still have your stereo effect, plus when you sum it to mono it retains the delayed sound.
Do you use and M/S plugins? As this could help as well. You could switch to the Stereo side and increase their level. Then when you sum to Mono this should help.
I've been using this plugin:
It has a great feature that sums the delays.
Here's the feature:
[i]WARP InputPan(new in version 1.0.1) -allows for reversed stereo image, mono summing of signals sent to delays, or left/right panning of the input signal. This allows for ping pong echos, as well as ping ponging delay clusters, tape echoes, reverbs, inverse reverbs, and sounds that don't have names yet.[/i]
You could also do some researching into the Haas effect and the Precedence effect. This will give you some theory behind the effects and steps to take to use it to your advantage.
It is all dependant on the instruments you use, the frequency area they occupy and os... There's alot of factors to take into account.
But after a bit of tweaking you should get a better result.
I hope this helps.