Transformer selection for CK260 CanaKit

I'm thinking about buying this kit to power a pair of 8Ω ceiling speakers (any good?):

50W Hybrid Power Stereo Audio Amplifier - Cana Kit

It's based on the Sanyo STK4432 amp IC:

The kit does not come with a transformer, so I need to find one that will work. The CanaKit site says I need a 2A AC transformer rated for 24-32 volts. Now granted the 25W/channel power rating is for 4Ω speakers I believe, so I probably wouldn't need 2A with 8Ω speakers. But I might use the amp to power some 4Ω speakers at some point in the future. I don't listen to music very loud (never above about 90 dB), so that's why I think I can get away with a smaller amp like this.

The places I buy from seem to like Hammond transformers. Here is their full catalog:

I was thinking one of the following:
166L28 - 28V 2A
166J33 - 33V 1A
167K30 - 30V 1.5A

Or go high-buck and get a toroidal:
182K30 - 30V 2.66A w/ parallel secondaries.

Now part of my concern is transformer regulation. Since I'm not going to be listening at full volume I'm guessing the transformer input voltage is going to be higher than the rating. However I don't know by how much since there's no regulation specs. I don't want the input voltage to be too high at low volume and damage the capacitors or diode bridge.

My other question is about the center tapped secondaries of the first three transformers. Is the rated secondary voltage between the center tap and the end tap, or is it between the two end taps? If it's between the center and end should I be looking at a non-center tapped transformer since I'll only be using half the secondary? Will the load imbalance create a problem if I don't use the third wire?

As you can tell I'm not very knowledgeable about transformer ratings so any help will be appreciated.