Building new system starting with power amp. What would you choose ?

 Personally I would never start with an amp. If already knowing the budget and room: I'd start with speakers.
 That aside, and playing along, if I were to start with an amp already knowing the lone source and without a preamp, i.e. RTR deck,: I would seek out an an amp with an input impedance, and sensitivity for maximum output that would suitably match the output impedance and voltage output of said lone source (RTR deck) with as much power as I might need. 

A pair of Classe DR3 VHC amplifiers used in monoblock configuration gives 100 watts
of Class A power per channel.  These were designed to drive the notoriously inefficient
Apogee Scintilla at its 1 ohm setting.  A used pair of DR3 VHC's should cost you
about $4000.  New in 1987 they were about $4300 each.  

Solid State: Ampzilla Monoblocks or PassLabs amps.  I have both.  Have the 250.5 PL amp- great sounding, presents solid state in a most pleasurable way.  Only problem is it weighs 100 pounds.  Just picked up a 150.5 PL amp.

Tube: Canary Monoblocks.  I have a pair of 140 wpc that were good stock and are outstanding after modification by Asi Teknology.  Decware Anniversary Mystery Amp.  If you have moderately sensitive speakers this 40 wpc amp sounds fantastic. I drove to Peoria, IL to listen to them.  

My experience is limited to what I have owned or heard elsewhere.  I know there are many other fine amps out there.

(Odyssey Audio)

You must chose the room first, not the amplifier, not the speakers....

No educated listenings ears give a dam about the gear, give me anything, or almost, and i will make it sound good...

It is called embeddings controls and treatment of the working dimensions of the audio system...

Any fixation on a piece of gear is an obsession, call it a taste if you want, but acoustic, mechanic and electricity are the 3 playing ground....

Read me right, i love my Sansui Au 7700, but the room/ears/brain is the main component....