Can a new amplifier affect speaker placement?

Absolutely yes, it can.

Each amp change I have made in recent years has required changes in speaker placement for best results. The biggest was moving to current Class D amps from prior Class A. That produced a startling change in soundstage, imaging and dynamics that required subsequent tweaks to get tuned back in.

The issue might be related to the nature of amp. The NAD is a digital amp, which I believe is called a class "D". The Creek is a hi-current type, probably "AB". From what I've been reading digital amps can be a bit thin sounding. Some of the positive points about "D" types are little to no heat and low electric consumption. The NAD might not be the best amp for you at this time. NAD makes more convention designed amps that are well reguarded too. You might want to look into one of those if you're looking exclusively for that brand - good luck.

Another way of viewing the problem: your new amp may be "showing" you that your speakers weren't optimally to begin with.

As Philjolet rightly points out, this is part of the fun!

How long have you had the amp? You need a good 100-200 hours of burn in before making any critical decisions. As stated synergy is a big factor the other amp could of had beefier or bloated mid bass which your speakers may be more sensitive to whereas the new amp may be tighter and deeper reaching which your speakers don't duplicate as much. every speaker placement formula i have seen is based on room size and response not the equipment itself. Moving a tad closer to the wall or a corner can beef up bass but this is usually associated with a unnatural bloat or bloom that is typically not desired. U can also try a power cord thats known for bass response in amps, tho my experience with power cords is that it makes a huge dif on some gear and very little on others, depends on design of power supply i guess c if u can borrow one.

Well, Stereophile specifications for each amp:

Creek 5350SE: Maximum current: >30A
NAD M2: Maximum output current: >27A

Could this explain what you're hearing? I don't know.

As stated above, what speakers are you using?