Yes it tightens it up, but does not boost the low ends like 6L6's do. I just put EL34's in my Recto and will never go back to 6L6's. They have a low end that is just to flabby and loose to my ears. The EL34's are more focused.
Do what I did and order a Quad set of Electro-Harmonix El34's for $40 on eBay to try them out. Then if you like them, get the better more expensive tubes. If not, you are only out $40 ya know? And here's a few vids from YouTube....
http://www.youtube.com/watch?v=iIvuBaDAPyk
http://www.youtube.com/watch?v=Q3BZPRG5Mj4
http://www.youtube.com/watch?v=yltbFMOMpWE