If you replaced the Pioneer with the Denon, I'd be surprised if you heard any difference.
As altpensacola points out, your speakers are a significant limiting factor.
Looking at the specs of the Tannoy SFX 5.1, the satellites are stated to play from 140Hz-78KHz, and the subwoofer from 45Hz-140Hz. That's not great for home theater, to put it mildly.
The Eltax Monitor Atmos is more in line with what you may expect. 70Hz-20KHz. They're probably alright for now, but if you had a way to mount them on the ceiling, firing down, they'd probably be more effective.
The Denon is "better" for at least having more channels (9, expandable to 11), better room correction (Audyssey XT32), and tested with superior audio output among AVRs, but you'll never be able to see the benefit of those.
If you're going to spend money equal to the cost of the Denon receiver, it's much better spent on speakers, because if you spent it on the Denon, you'd still have to spend on speakers.
Find a place with an easy return policy or risk-free trial and see for yourself. Depending on where you're located you could probably see a huge difference spending half the cost of the AVR on speakers (given the models of speakers, it suggests UK).
I say half on the speakers because you'll want to spend at least half on the subwoofer!
If the majority usage of your system is HT movies, as opposed to music, IMHO the sub can have the largest impact (literally) in HT, then the center channel, then whatever you can do for the rest.
Originally Posted by Gavilav
I currently have a Pioneer VSX-933 using a set of (old) Tannoy SFX 5.1 speakers and 2 Eltax Monitor upward firing speakers so I can have a 5.1.2 system and play Atmos where available.
Is the Denon a significantly better piece of equipment? Am I likely to see a noticeable increase in quality?
Originally Posted by altpensacola
I would not blame your sound issues on that Pioneer when using those little speakers. If you really want home theater
look at investing in bigger speakers. Even if you only borrow a left/right/center set up to test I bet you would find out what that Pioneer can do.