Tubes 101

B limo No all 6550's or KT88's do not carry a specific sonic signature. Example a SED 6550 is very linear sounding where a original Tung Sol 6550 black plate is better in the bass and have a midrange to die for (similar to EL34's but better highs and lows). EAT KT88 is also extremely linear with a lot more detail than a SED 6550. The SED KT88 is a lot 'warmer' sounding than the SED 6550 with the Gold Lion KT88 re-issue somewhere in between. The original Gold Lion KT88 is also somewhat linear but a tad dry sounding. Ei KT90 type 2 is has an excellent mid range huge soundstage with great highs and very good lows.

That being said the topology of your amp also plays a part in the overall sound. The little input and driver/splitters also make a difference. Honestly you need to try them and form your own opinion. If the tubes are new give them at least 50 hrs. Some tubes take up to 300 hrs to settle down.

B_limo - what tubes are in your amp now? What change would you like to hear? Are your current tubes new or pre owned? If used give them 25 hrs before making a judgement. If new they could take 100 - 200 - 350hrs before they settle down which is why I asked what tube are in there now.

B limo
I have SED 6550 and KT88's. Not sure how close the Svetlana sounds compared to SED. But from what you are describing I would suggest Ei KT90's (type 2 or 3). Besides great extension at both ends it has great mids too not to mention a huge soundstage. They are a little pricey but nothing like original vintage 6550's or KT88's. I also have no experience with EH but a local tube seller prefers the Ei KT90 over the Eh KT90 by a long shot. Finding Ei KT90's can be a little difficult since production was halted in the early 90's. Ending in a few hrs - no affiliation
http://app.audiogon.com/listings/tubes-kt90-ei-nos-amazing-sound-kt-version-ei-kt90-second-generation-quad-2-spares-new-in-2014-01-04-accessories-33441-deerfield-beach-fl

Jburidan - Per Jim McShane (one of two local tube sellers) the type 3 is a wonderful tube as long as you didn't get the last from the factory. The last ones had mis-aligned heaters which caused the anodes to get cherry red (typically in a few hrs) before arcing over resulting in premature failures. Probably not many left. I have 2 quads of type3 and 3 quads of type2. Except for the type3's being slightly lighter (thinner glass) and the lack of spot welds on the anodes they sound the same to me.

So I do agree there may be a handful of bad type3's still out there there are a lot more good ones left. The type1's were the worst by far.

B_limo don't get too ahead of yourself swapping tubes. For any advice to be meaningful you need to know what you want to change the sound to with your current tubes. Looks like the PL5 comes with Gold Lion KT88's for outputs. But what brand are the 12ax7's and 12au7's? Also the PL5 is autobiasing so another word of caution before swapping output tubes. You want to get matched quads. Unlike manual biasing the autobias has a tighter window for biasing.

Where are you that you are snowed in?