Different Tubes and general purpose?

Tube gear manufacturers tend to supply their products with decent sounding, but lower cost tubes that they can procure a large enough supply to fulfill new orders and to have backup tubes for customers wanting them to supply replacement tubes.  Availability and price are key considerations.  That is sensible because any more exotic and pricey tube MAY, but not necessarily will, improve the sound--improvement is a matter of taste and system matching.  That leaves it up to the buyer to decide whether a particular alternative is an improvement.   Ultimately, it comes down to the owner having to either purchase or borrow candidate tubes to listen to the results.

Because trial and error is really the only approach, I would recommend starting with the cheaper small tube types.  If you are new to tubes, go slow and enjoy your amp for many months before attempting to change the sound through tube swapping .  It takes time to become familiar with the sound so that any change makes an obvious difference.  Also, expect to make mistakes--where a change initially sounds good, but in time turns out to be the wrong move.

Most people who have been around tube gear a while can pretty readily hear differences between tube brands and models.  Like any form of connoisseurship, experience matters.  If you are new to wine, it is hard to distinguish, much less reasonably evaluate which is better, without experience and experimentation.

Good luck.

Buy a book dude!!

Thanks all for the practical advice.   
I’ve had my amp for 2 years now and it probably has 50-60 hours of use, so I would have thought it is been broken in.  I will call my distributor and ask what he thinks in terms of where to start on rolling any tubes.  I’m not in a rush, so I have time to play around. 

Tubes will make a big difference. All you have to do is swap in some NOS tubes from back in the 50's or 60's to more current production tubes and you will hear why these NOS tubes are superior. As one of the prominent vintage tube seller's said, "I would rather have a good amp with great tubes than a great amp with merely good tubes." 

Anyone who tells you to use fuses, powercords, or conditioners in place of changing to superior tubes is giving you misinformation. Don't get me wrong, those things can give you some improvement too, but really good tubes speak for themselves. You should give it a try. IMO.

gunners01

I have the Line Magnetic 518IA integrated , the power tubes in mine are 845s versus your amp's  KT120s. I left in the stock 845 but changed the 6L6s to NOS GE, the tube rectifier to NOS RCA and the 12ax7s to NOS Tung Sol 5751s. The NOS tubes took the sound of the amp to a more satisfying level. For me, I found the lower gain of the 5751s more pleasing in the 518IA. Others have noted some NOS recommendations for 6sn7s, if you want to try new production I'd suggest either Sophia Electric 6sn7s or Black Treasure CV181s; I have used both in my Modwright LS100 preamp and found them quite good sounding. Per jetter's post it would be worthwhile to you click the link and  read Brent Jessee's assessment of the different individual tube types and their respective variants. So while I disagree with millercarbon's assessment of tube impact, I will agree with him that a quality power condition and cord, and isolation feet and so also benefit my equipment. I haven't gone the fuse route so I can't comment on that one way or the other.