I think its absolutely worthwhile. Ive had the opportunity to try out 4 types of tubes in my unit and would say each type has different merits. Out of the four I would rank the jj tesla bottom - they sound good but in comparison they seem a little 2 dimensional.
My amp came with a matched quad set of philips SQ, and these are a very nice tube. Very warm, extended bass, and smooth treble, they sound liquid!
Recently curiosity got the better of me and I snatched up a deal on ebay for another set - 4 matched quad Mullard cv2492. These turned out to be the Ediswan military grade tubes, and again they are a big improvement on the stock. They are very airy, not quite as much bass as the philips, but a wider sound and great detail on the mids and treble.
I also came across a matched quad set of siemens A-frame. They have a date stamp from the 60s. Unfortunately they are microphonic, the first time I have experienced this phenomenon, with the volume turned down you hear every slight tap on the amp cabinet magnified through the headphones. With the volume up, the music takes over and everything they say about the Siemens tube is true. It is just a marvellous all-rounder. Detail, width, attack, bass to treble, they have stayed in my system since. 'Attack' is where I would place the Siemens above the philips, my music seems to come through a little more punchier. (Im making these terms up now lol)
So there you go, yes, I whole-heartedly agree on an upgrade of the e88cc stock tube. Unfortunately they are expensive to buy from a reputable dealer, but there are deals out there if you are willing to negotiate the minefield that is ebay. Just keep an eye out, be patient, and do your homework so you know what to look for. There are pleeenty of fakes out there.
And yeah probably best just to get the Siemens.
