My nickels worth- You are on a path that is perfectly normal to upgrading your sound. Adding an amplifier is an excellent first start and you should not feel like it was a bad decision. It was money well spent. It will increase your relative volume output resulting in lower gain settings on the processor, because the amplifier is probably more sensitive to the input it is receiving and more than likely is a lot more efficient. It will not dramatically enhance your sound or provide greater clarity / sonic characteristics. Overall the amp is going to take exactly what you feed it and spit it back out. A good amp will do this without introducing additional qualities, although there is a discernible sonic difference between tube based amps and digital / solid state amps- Which is why pure 2ch audiophiles go for tube amps and home theater dictates the use of digital / solid state amps.
In order to achieve marked improvements in your sound, other measures are necessary such as moving to a better quality of transport, processor, and speakers. For a rough example, you could have a $900 preamp/processor with built in amplifier, $900 speakers and mid-line cables / interconnects, and then add a $10k Theta amplifier. The system will sound pretty much the same with the Theta, but of course there will be more power on tap. The Theta is sitting there waiting for a comparable quality preamp on one and and speakers on the other end in order to really shine- But at that point all it is really doing is transparently reproducing the high quality input out to the high quality speakers.
I'd probably suggest a higher end processor as your next upgrade as it will give you the most readily apparent sonic improvements. Transport next. Speakers next after that, if warranted.
My upgrade path from an all in one receiver amp was as follows:
1. moved to a separate amp (minor difference noted here)
2. moved to better cables / interconnects (no difference noted here)
3. moved to better / dedicated processor (Big difference noted here- on everything)
3a. tried different processor, better on surround but horrid on 2ch
3b. tried different processor, better on features and sound in all respects, traded in 1st processor and kept this one.
4. moved to better quality amplifier (no difference noted here)
5. moved to better quality DVD and CD transports (minor sound difference, major picture quality difference)
6. moved to better speakers (martin logan / energy / REL combo) - Big difference noted here in sound, spaciousness, bass, etc.
7. moved to better preamp / processor - minor difference noted here, better channel separation and effects for movies, slightly cleaner 2ch music, better / newer features, allows me to convert all interconnects to XLR. No real changes in terms of sound spaciousness.
8. moved to synergistic reference speaker cables and XLR interconnects with active shielding, and added a TrippLite AVR for entire system. - (slight audible difference in 'blackness' of sound, cleaner picture for TV and DVDs)
9. moved to Genesis 6.1 speakers - huge difference, gains from all above changes now realized. Extremely spacious sound, the ultimate.
High end speakers were the last step of my upgrade evolution path- I was quite pleased with my Martin Logan fronts / centers, Energy surrounds and REL Storm subs, but was a bit puzzled why adding vastly better components did not make much difference in overall sound quality. Once I swapped in the Genesis 6.1 speaker system the difference was night / day astounding- With the same exact gear. At that point, my previous speakers were the limiting factor and while my pre/pro, amps, interconnects etc were extremely high quality, I was not able to realize those gains until I added some speakers that could play back the sonic improvements. When I was starting out and just got to the point of adding a separate amp, upgrading the processor after that gave me the next big leap in sonic improvement. The whole point of this is that once you upgrade a component to a higher quality, you often don't get the full benefit of it until all other pieces of the chain are at the same quality level or higher.
On your other item:
For surround speaker placement, higher and 8 or more feet away is typically better. surrounds are there to provide ambient effects with some localization, so having them at a distance to maximize that effect would be better. If possible, do a wall mount with them about 3/4 up to top of wall or all the way at top of wall, angled slightly down towards your listening position. Surround sides should be positioned to the direct side of, or just slightly forward of the primary seating. Sometimes slightly behind the seating works as well, it depends on the room and what type of speakers you have (monopole, dipole, line array, etc), and if you have rear surround speakers or not. Typically setting the surrounds on a stand at couch height is not the best setup.
