Amp first approach to building a system, anyone ?

With one exception. If your main source is going to be RTR high end deck. Then that's where you start. You can also take it with you to audition the equipment. Don't rely on digital source only and even turntables unless it's top level. What will sound great with your perfectly working deck is what you will want.

Yeah, there is no true audiophile system without excellent active preamp, everyone knows it.

I would first select your speakers as that choice can determine your choice of amplifiers.  Very efficient speakers don't require the high power and speaker impedance has some to do with amp selection too.  Efficient speakers with relatively higher input impedances can be driven easily with flea-watt amplifiers such as 300-B tube types and First-Watt low wattage amps for terrific sound.  When I moved from 98db/12.9-ohm speakers to 88db/4-ohm speakers, I had to seriously change amplifiers. In any event, you should choose an amplifier that can provide headroom of ~ 10db above your higher listening levels to allow for high transients (headroom) reducing the risk of clipping.  

My long audio journey over the past 50 years started piecemeal to take advantage of roommates equipment also, so actually first purchase was a TT. Then gradually building system all my own. Then many many individual piece upgrade or experiment.

Cant say I am amp first, though after rolling through several, I have repurchased one of my previous that I end up preferring.

like @pindac  I would say I am polygamous. I have both SS and tube amps, 2 phono preamps, and 2 pairs of main speakers (of different designs).

I find I can satisfy my need for variety while also have well-matched components.

(Also have a herd of vintage components)

I like single ended triode amps, but I do not want to deal with speaker cabinets, which are engineered as the famous flying wing aircraft designer put it, "invent rubber gloves to write with leaky fountainpens." That limits me to planar magnetic speakers. I use Magnepans which sound better than a pair of $50,000 + speakers that have cabinets. But they need mor than the likes of 300B SET's can produce. 

I took 46 bulb SET's and used Lundahl interstage transformers to go from 5k to 600 ohms that I coupled to the grids of 833A triodes run at 1000 plate Volts and Hammond output transformers to Magnepan 0.7's and bass panels. The 833A needs the purest DC for the 10 Volt 10 Ampere directly heated cathodes. You can do this modification for about $1500 or less and it will have better parts than a WAVAC 833A pair of monoblocks costing $480,000 or more as inflation takes over.

Common acoustic sense is this ideal order:

 1---Room geometry,topology acoustic content and parameters first....

2---Speakers  must be adapted to the room and chosen location and picked accordingly ...( you dont put 5 inches subwoofer  in a Trump ballroom) 

3---Amplifier exist to serve the chosen speakers mainly then third...

4---Dac source or turntable must serve the amplifier and the system/ears then  last ...

You can buy the dac first for sure, i spoke about ideal order of electrical and acoustical synergy ...

I suppose if someone is in love with his speakers they were chosen for a specific room and needs, the same is true for a pair of chosen amplifier serving a set of speakers... Changing one will imply a choice determined by the one we kept ...

What cannot change is acoustics, the link between the room and the system to your head/ears, is the only link that guide us all ...

Then anything else matter less ...Even our "taste"  for a brand name speakers or amplifier is secondary because we cannot know the sound qualities of a system/room before we experimented with it ...

The ultimate rule i discovered in audio hobby is this :

Any picked  system/room sound completely different on a quality scale before and after the implementation of the  tetrad of factors entering into the optimization process (mechanical,electrical,acoustical&DSP ) 

Then this rule has nothing to do with "taste"  but everything to do with basic knowledge... Anything else is the result of marketing conditioning ...

 Once this is said we all have our "taste" born from our own audio history of purchase... I like my Sansui amplifier and i miss my Tannoy speakers...