Joining speakers with 1 amp?

One speaker (components w/crossover, coaxial, or single) per channel.  Doubling up speakers that supply the critical front end of a sound system to get more power out of an amplifier is causing the amp to work harder to provide greatly inferior specs.  The result is inferior sound.  Power the inside speakers at 4 ohms with amplifiers that are powerful enough to provide that power cleanly.  So, plainly, there is a difference.

Look at several amplifier specs.  Look at the output at 4 ohms impedance.  Then look at the output at 2 ohms.  In most cases the output is double at the lower impedance.  But the specs given are at the 4 ohm rating.  2 ohm loads are acceptable, generally, for sub bass output only.

See info for parallel and series wiring and the affect those wiring configurations have on the final load impedance from many sources;  a search of this forum will provide plenty.  And BCAE1.com is good for all the basics.

You are hung up on power and ohms.  Neither makes a speaker sound good.  A speaker either sounds good or it doesn't.  Buy the ones you like.  Give them sufficient power within their RMS rating. 

In general, however, any amplifier will operate cleaner, cooler, last longer and sound better with a higher impedance load on it.  This is generally true no matter what kind of speaker we are discussing.

Overpowering kills speakers.  Underpowering does not harm speakers.  Amplifier head room is a good thing.

Like I said before, in general an amplifier will operate cleaner and your sound quality will be better with a higher impedance load on the amplifier.

The difference in loudness between a speaker using 80 watts of amplifier power and the same kind of speaker using 160 watts of amplifier power is around 3db.  (as a point of reference, it requires an increase of 10db to be perceived as "twice as loud." ) 

Use the speakers that sound the best to you.

The gain control does not limit the amplifier's power output, it only determines how hot an input is necessary to achieve the output.  An amplifier is capable of it's maximum output with any gain setting.  The purpose of setting your gain properly is to prevent amplifier clipping and to balance the relative output levels between multiple amps at any fixed input level. 

As far as how much amplifier power for a given speaker, actually you can use any amplifier with any speaker.  The key is understanding how to set up the system and how to recognize and correct clipping or speaker over-excursion issues.  Generally, though, I recommend that if someone is purchasing an amplifier that they use any amplifier RMS output up to the RMS ratings of their speakers for the safest operation, and to be sure they understand how to properly setup the amplifier to prevent clipping.  Remember that speaker ratings are the MAX they can safely handle, not what they require.