It is possible to do everything from one manufacturer e.g. McIntosh (except for the turntable, cartridge, tonarm) but you will sacrifice quality in certain elements. e.g. Mac's amps and preamps are excellent, but IMO, you could do better with a CD player and speakers from other manufacturers, etc.
That said, for starters, I've found that grouping certain system elements, according to excellence in a particular area, can yield a successful synergy. Using my own system just as an example, Wadia for digital, Levinson for amp+preamp, MartinLogan for speaker/subwoofer, Purist for cabling, etc.
On the other hand, I recently replaced my (very excellent) Levinson amp with a Mac tube amp, just because I prefer tubes for driving electrostats. Yet I don't intend to replace my Levinson preamp with a Mac preamp (any time soon;--) just to maintain uniformity of brand in the preamp+amp group.
However, grouping things this way can be a good starting place, and is possible even within a modest budget.
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