Let's start with the tubes: EL84 and it's Big Brother EL34, are members of the Power Pentode family. A pentode has 5 active electrodes: cathode, anode (plate), control grid, screen grid, suppressor grid. The grids control electron current flow from the cathode to the plate. The control grid has the actual signal on it, the screen grid helps attract electrons which then slings them toward the plate, and the suppressor grid helps gain by returning electrons that literally bounce off the plate at high velocity. Despite the three control grids, the electrons reach the plate in a more or less diffuse pattern, and some of the electrons get lost, attracted back to the screen grid, where they cause an increase in screen current. This causes power pentodes to clip easier and in general, have a bit of a softer sound and slightly lower gain, not as edgy or glassy as the 6V6 or 6L6, which along with other famous tubes such as the 6550 and KT88 et al, belong to a family of tubes known as “Beam Tetrodes”. Tetrode means “4 electrodes”, not 5 as with the power pentode. In beam tubes, there is a set of “beam plates” after the screen grid. These are connected internally to the cathode, hence not considered to be a separate electrode, but part of the cathode.

As the negatively-charged electrons zoom past the screen grid, they go between the beam plates. These plates, like the electrons, are negatively-charged, since they are attached to the electron source, the cathode. This forces the eletrons together in a very tight, efficient pattern where they are concentrated and blasted onto the plate shotgun-style. This sharp, focused field is what gives beam tetrodes their characteristic focused, sharp tone. Easy, right? It also allows the tubes to produce more gain and in general take more abuse. Power pentodes often have screen grid meltdown. Screen grid failure is the most common failure mode of power pentodes. As the tubes are pushed to their limit, more electrons bounce off of the plate and back to the screen grid, raising screen grid current which cannot be drained away fast enough. Eventually, the screen grids will short and the tube is toast. Beam tetrodes do not have a screen grid, hence can take the punishment. This is why most high-power tube amps use beam tetrodes. The 6550 and KT88 are the kings of the beam tetrode family, and can put out a respectable 50W each! So, if you see a 100W Marshall, for instance, with 4- 6550's, those 6550's are putting out a very cushy 25W each, and should last a LONG time, longer than a set of EL34 in the same amp.

Now, to your amp: there are many design parameters that make an amp sound the way it does. If the Mesa works better for you, that's great, but it's not just the power tubes. Mesa and Fender preamps are world's apart, and my guess is that you like the Mesa preamp sound better. Actually, there is not much disparity in power output between the EL84 ad 6V6, but the 6V6 will always sound louder because of the way a beam tetrode responds, i.e. quick and direct. The EL84 with always sound “rounder” and more subdued.