Nothing to offer except congrats on the incoming album. Be sure to post it here when it’s released.
When reading your post it seems like the sound engineer tried to give some common sense feedback on your mix. Sounds that are not the main the focus of the tracks sitting in the middle of the stereofield can totally obscure the lead track.
Reading your version of his comments it also seems like he was not commenting your synth. And digitone is perfectly capable of being used for wide sounds.
To me it seems like he was saying that with your recorded material it is gonna be hard to separate the vocal/lead line and put it up on front.
My best advice is contact the sound engineer and find out what he meant. That seems more productive than asking forum members to help guess what he meant.
Also, from my experience playing in a big band with keys and synths, sounds can become quickly way too much for a decent mix.
This means that for in this band I have very “thin” versions of my sounds that work really well in a busy mix… They cut through and give space. But in isolation these sounds are kinda awful.
Also, as said above, be careful with too much stereo tricks. This can indeed very quickly lead to issues with phasing and such. Especially playing live on a big pa. So for my sounds I hardly ever use stereo and I am very gentle with any spatial sound processing.
And to add: those thin mono outs I do pan very hard on the mixer to get the stereo width and separation from the vocal thats needed.