An honest opinion about upgrading?

Are your speakers on good stands? Also I would experiment with cones under speakes and equipment. Reduce room reflections behind your speakers.

I would upgrade the preamp first, probable to a good tube unit

For definition and clarity, maybe worth thinking about the mains installation, and running your system from a balanced isolating transformer, i.e. equitech, well respected in the us. This made quite a difference to mine, i was happy overall tonally, and i listen to similar music to yours.

Your power amp is not well suited to drive these speakers. S-8 has very low impedance of around 2.5 Ohms. I'd get some serious power if you want more clarity

IF the volume levels which you are obtaining are satifying to you; much of the improvement you desire can be had via NOS driver/phase splitter tubes in your power amp. For definition and clarity, I'd personally recommend a pair of Siemens ECC83's, and bottom gettered Sylvania 6SN7GT's, from the 40/50's. The same 6SN7s can be used in the pre, but they have to be graded, "low noise". Some examples of what to look for, that won't break the bank: (https://www.tubeworld.com/12ax7.htm) (http://cgi.ebay.com/Radio-Tubes-1-Sylvania-6SN7GT-6SN7-Tube_W0QQitemZ350241000213QQcmdZViewItemQQptZVintage_Electronics_R2?hash=item518bfd8b15&_trksid=p3286.m20.l1116) Installing dedicated power lines from your mains(I used two parallel runs, twisted, of 10AWG) will improve your system's dynamics. A pair of Synergistic Research AC Powermasters on the pre and main amps will offer greater definition and imaging improvements(If the speaker/room interface is up to it). Kimber Heroes, Silver Streaks or KCAGs will also help you achieve your goals, as they are more transparent than many other cables in the price range you mentioned. Where were the output tubes manufactured? If Chinese; you may consider a set of Winged 'C'(St petersburg plant) KT88s, ie (http://www.upscaleaudio.com/SED-quotWinged-Cquot-KT88_p_4.html).