I always thought the reverse was true, and that 6l6's were a "cleaner" tube...?

-D

6L6 Tube:
When Fender began producing guitar amplifiers, they based them on an RCA hi-fi design. RCA gave free use of these designs to promote the use of their tubes, in this case, the 6L6 power tube. The beam tetrode 6L6 has a round scooped voiced uncolored "white" tone that is crystal clear in clean and a Bluesy overdriven style of distortion when pushed. The American 6L6 makes up a large part of the signature Fender sound. In comparison to the EL34, 6L6 tubes have slightly less output.
6L6 tubes are best suited for those using mostly or only guitars with single pickups, like a Fender guitar through Fender amp sound. 6L6 tubes and single pickups are both scooped voiced which compliment each other's sweet spots. Humbucker / P-90 pickups will retain the full-bodied tone in distortion, a common combination for traditional electric Blues like BB King and Buddy Guy. In clean, Jazz through a Fender style tone, muddiness can be a problem. One solution for this is higher wattage speakers.
"British" amps will lose much of the signature Marshall grind but it wasn't unusual to use 6L6 or similar tubes in Marshall amps. A notable example is Jimi Hendrix. Jimi switch out the stock British made Mullard EL34's in his Marshall Plexi with American made Tung Sol 6550's. 6550's sound like 6L6's except much louder and they tend not to distort when pushed. Jimi did this to have his Marshall better fit the tone of his Fender Stratocaster.
6L6 tubes are also known for high gain distortion. Though the 6L6 is mostly associated with Fender powered American Blues, Country, R&B, etc, it was also used in the original high gain amps (Mesa Boogie "Mark I" & Soldano "SOL 100"). To solve the problem of extreme muddiness in high gain distortion, the mids were dropped out. The 6L6 naturally distorts in a scooped voiced fashion.

5881 / 6L6WGB Tube:
The 5881 / 6L6WGB has slightly less output then a 6L6 with an earlier break up. These tubes are a favorite in Fender style amps to distort at a lower volume. 5881 is the military designation and 6L6WGB is the commercial designation.

EL34 Tube:
Marshall originally had American 6L6 tubes in their guitar amplifiers back when the JTM45 was essentially a Fender Tweed Bassman clone. After deciding that importing American tubes were too expensive and that close was good enough, Marshall switched to the domestic 6L6 British equivalent, the GEC KT-66. After finding this too expensive as well and desiring something with more output, they moved over to the British made Mullard EL34 for their "Plexi" amps in the late 1960's. It is the EL34 that has defined the sound of British Rock.
The pentode EL34 has a midsy crisp sound with a grind crunch distortion when pushed. The EL34 does not have the crystal clean sound or the headroom of the 6L6. However, because of the higher output, you can run an EL34 clean at a higher volume then a 6L6.
The EL34 is best suited for guitars with humbucking / P-90 pickups, like a Gibson through a Marshall sound. EL34 tubes and humbucking / P-90 pickups both have a midsy tone that compliments each other's sweet spots. EL34 tubes work also well for those using single pickups, though it will have a less full-bodied tone then with 6L6 tubes. A notable example of the typical "Les Paul Gibson through an EL34 equipped Marshall" setup is Angus Young from AC/DC.
The modern EL34 is a more versatile tube then the 6L6, easily usable with single pickups and humbucking / P-90 pickups in clean and distortion. EL34 tubes are not known for their clean tone but it can remove some the muddiness in humbucking / P-90 clean tones. Though most Jazz guitarists using a tube amp will use a 6L6 equipped Fender style amp, EL34 tubes can offer more "spacing" between the chords, making it easier to EQ.

6550 & KT-88 Tube (high wattage):
The beam tetrode 6550 tubes are tonally similar to the 6L6 with a scooped voiced crystal clean sound. The beam tetrode KT-88 is tonally similar to a KT-66 with a warmer, less crystal clear, sound then a 6550. In a Rivera, four 6550 or KT-88 tubes will produce about 150 to 160 watts. Two 6550 or KT-88 tubes will produce about 75 to 80 watts.
The most common use of 6550 and KT-88 tubes in amps is for guitarists that need a louder clean tone and do not require a full-bodied distortion tone. When pushed, these tubes do not produce the harmonically rich distortion like lower wattage tubes but tend to stay clean and just get louder. With any increase in wattage, the tone will become brighter, deeper lows, and have a lesser emphasis on the mids. The brightness can easily be compensated for by the use of darker preamp tubes.
Two other reasons guitarists use high wattage tubes:
First, guitarists seeking to get a Jimi Hendrix sound out of their Marshall amp use 6550 tubes. Jimi switched out the British made Mullard EL34 tubes for American made Tung Sol 6550 tubes. He wanted the tone of a 6L6 to better fit his Fender Stratocaster yet as loud as possible. Jimi did turn up his Marshall amps to 10, but much of the distortion was derived from the preamp section or a fuzz pedal. For those not performing in stadium sized venues, a 6L6 is a better alternative.
Second, shredders like to use 6550 tubes in their high gain amps for the tight exactness to play a billion notes a second.

In regards to the first post, Marshall : "their sound is saturated and not so mid-heavy". This is not right. Marshall's are all about the mids.

Actually i think that was right about marshall.

http://www.engl-amps.com/engl-amps/ for ENGL amplification,most people whom have played through them love em,those that have'nt seem to dismiss them as cheap mesa copie's

For what it's worth,i'd have prefered a Cornford or Tri-amp,but budget allowed me to get My ENGL,and i have not been dissapointed once,an by jesus for a 50 watter is frickin loud :-)

Rob

ENGL is better than Mesa any day IMO

Another thing regarding amps never believe brand names or magazine articles. Some amps (not metioning names) have too much hype. When cheaper amps are just as good.