I don't know that I am steadfast in that sense, because overall I think 'balance'in the chain, though if pressed I'd say source. The reason for the source-first camp is the old adage; garbage in = garbage out. If you start with garbage as the source, there is absolutely nothing down the line you are going to be able to do other than make the garbage smell a bit sweeter, but it's still garbage when it comes out the end. So if you take a source that is not capable of nuance, that is fatiguing, lacking in detail and soundstage, and harsh in the highs, I don't care what you put in the rest of the chain...you can have speakers that look like screen doors, or public waste cans, or giant horns from Dr. Seuss...you can have amps with enough power to light a city, preamps with glowing NOS tubes from the 1950's, and cables that cost enough to feed a family of four for a year; you can have all this stuff in place and it can be working in perfect synchronicity in a room that resembles a miniature world class concert hall and is acoustically perfect..you can add your clever lil' clocks and magic bricks and lead-weighted everything...you feed that crappy source through that otherwise perfect system and you will be treated to harsh highs and no soundstage and you will be fatigued from listening to it at length. You feed the signal from a Close-N'-Play turntable throuth that system and it's not likely to bring any smiles other than smiles of mild amusement at the novelty that you made it sound a lot better than it should. But it still is not going to be something you'll want to keep listening to. Reverse the situation and put a world class front-end with a world class middle components, and crap speakers and I'd bet you could listen and be amazed for a whole lot longer than compromising the source. These are extreme examples to illustrate the point, but the fact is it's all connected in a chain and you should go for balance and strength throughout. That said, I do think that speakers do make a big difference in the kind of sound your system produces...the sonic signature perhaps. This is an endless debate, repeated over and over here. There's my .02 lincolns again.
Marco