Wilfredt,
IME, it's hard to answer your question on a "tube vs tube" basis, because so many other variables are changing, too. (I know of no amp that allows the user to swap KT-88s and 845s for an "apples to apples" test.) From the circuit (per Phaelon) to the partnering speakers - the resultant change in sound upon changing from a KT-88 to an 845 may be hard to predict, and in any case, would be hard to attribute solely to the output tube.
I have used both the Cary 805 (w/845, briefly) and the Prima Luna mono 6 (w/KT-88, briefly) with my Verity P/Es and my Merlin VSMs. (Ironically, I ended up ditching both the KT-88 and the 845, using 211s long term in the Cary and KT-66 long term in the PL - see below.)
The best short answer that I can provide to your specific "punch" question is that the KT-88 definitely struck me as significantly "punchier" on the Verity (tho I prefered the Cary, overall). On the Merlins, the delta in macro-dynamics was much less evident.
Overall, the 845 feels like a somewhat darker, more liquid sounding tube than the KT-88 to me, but that observation is subject to all of the caveats noted above.
Marty
Note: My Cary uses the 845 in an SET configuration, which may explain the difference in sound better than the output tube delta. The Cary also allows the user to swap a 211 for the 845 and (with the Verity) this seems to reduce the slightly "dark" character of the Cary amps, so I switched to 211s for long term use with the Verity. Conversely, the KT-66 struck me a somewhat "gentler" sounding than the KT-88 in my PL, so I ended up going that way for use with my Merlins. In the end, all output tube decisions were system driven.
