Alternatives to McIntosh MC402?

Keep the MC402. Buying a different amp is a sideways move at best. Your MC402 will be working well for another 20 years! All well-designed amps operated below clipping will sound alike. If you don't drive the MC402 often into clipping there is no need to change!

Someone will soon text "try a different power cord".

Sounds like your MC402 can handle the wide dynamic swings of large orchestral recordings without clipping. 

If you want a light weight amp with plenty of power a nicely restored Carver 1.5 will fit the bill. 450 wpc into 8 ohms. And it won't cost $5K!

For difficult-to-drive speakers I have a Perreaux 2150B. 340/680/920wpc into 8/4/2 ohms. Bought on eBay for around $900. I turned down an offer to buy a big Levinson amp for $3K as the Perreaux is sonically as capable as a  Levinson.

I used to own Olympica 3 for many years and now own Amati. They are pretty easy to drive. My preferred match is Audio Research. While warm and extremely natural they are more balanced like real music than Mac (it will take away the dark veil). Provided your musical emphasis is not at head banging levels, I would check out a used Audio Research VSi 75. I am using a ARC Reference 160S.. but in triode mode only 70 wpc and they can play much louder without distortion than I would ever consider playing them. 

The other difference is you will get more natural and nuanced bass… plenty of it. But, not the artificial slam typical of solid state amps. 
 

If you choose solid state, then you will need a lot more power. Consider a used Pass 150watt amp.

Maybe a MC312? This is what I use in my 2C system, driving Magico A5 speakers. Very happy with it, stunning sound quality streaming, vinyl and best of all, 15ips master tape copies.

Thank you all for your comments.  Audio Research is intriguing, and ghdprentice's advocacy of the AR/Sonus faber combo on this forum (and in other discussions) is rather persuasive.  But I would rather not fuss with tubes, and I don't want a space heater -- I live in Florida.  I also have reservations about the adequacy of power.  My spouse and I frequently listen to demanding music at realistic, concert hall levels -- i.e., loud.   I think I need some power to drive the Olympica III's adequately. I haven't actually heard the Audio Research; if I came across a great deal, I might not be able to resist.  ("I can resist anything except temptation," Oscar Wilde wrote.). 

The MC312 is an amp I'd probably like, but the main reason to move away from my MC402 is that it's heavy, and quite difficult to move at 110 lbs, -- and the MC312 is 105 lbs, so the difference in weight is minimal.  

Pass is interesting, but I have a question -- how warm do the Pass AB-class amps run? 

There are a couple of other possibilities -- Musical Fidelity amps (such as the M6i, which puts out 440 watts at 4 ohms IIRC) and the Anthem STR, another integrated with a similar power rating.  (I'd consider either an integrated or a power amp.)  Does anyone have thoughts (or experience) as to how these amps compare sonically with the McIntosh MC402, or work with Sonus faber speakers?

Pass amps do not run hot at all. I ran a X350’s for many years. They would never get more than slightly warm. I recommend either a contemporary XA series or X. The X series have been getting better rhythm and pace in recent years. 

Explore:

Couple Bryston 4B cubed amps on here for a little less than $5K each. that be a different sound , powerful. I think it comes in around the 60 lbs mark. 3B would be even lighter. Couple PS Audio M700 mono blocks would weigh even less (26 lbs total) and plenty of power as well.  Pair of M1200 over on USAudio as well