I would have to assume that it would help a little. I havent tried EL34's in my recto yet, BUT....I can tell you that from personal experience, I find that harmonics do stand out better with more mid/upper mids present. Not that this amp wont do pinch harmonics.....but they are a little more resistant as compared to other amps Ive owned/played. They just dont jump out as easily, and it seems that you do have to be a little more careful in where, and how you play them if you want them to stand out. This would make sense considering that the rectos are light on the mids, and adding mids in any way seems to help in my experience.
Recently, Ive added an MXR 10 band and an OD808 to my rig, and with the extra mid character of the OD, and the mids that I dial with the 10 band.....I find that among other things, harmonics now stand out much better. There isnt a sweet spot anymore.....they jump out anywhere I want them to. FYI, this has been my experience with various low and high output pickups. Id expect an EL34 with good mid character to have a similar effect.