Modified Adcom 5400 or Stock 5500?

All other things being equal, I'd go with the 5500. The Maggies usually perform better with more power, and I think the general sound quality of the 5500 is better to start with than the unmodified 5400. You can always have the 5500 modified at a later date to improve the sound quality, while it would be impractically expensive to have the 5400 modified to equal the 5500's power output.

I use the 5503 (3-channel version of the 5500) with MG1.6 that are crossed over from subwoofers at 90 Hz. Results are excellent. The difference vs a well-regarded low power Adcom amp with similar specs (100 watts vs 350 watts at 4 ohms) is dramatic.

Great bang for the buck.

Art's comments spurred me to add that I have owned both the 5500 and 5503 amps, and gave both of them to my son when I upgraded by own system. My son previously had the Adcom 545, which has about the same power output as the 5400, and when it was replaced by the 5500, there was a significant improvement in the sound of the system.

Checkout some of the 500 series Adcoms .They will do a better job driving those Maggies than a 5000 series. The bipolars are better for low impedences and put out more current.Don't be fooled by the watts.Take for example the 5802 is 300wpc into 8 ohms and 450wpc into a 4 ohm load.Maximum power consumption 1440 va.Ok now the 585LE 250wpc into 8 ohms and 400wpc into a 4 ohm load.Maximum power consumption 1800 va.5802 slew rate is 90 volts ,585 is 120 volts. A 565 or 585 will put out over 100 amperes per channel.None of the newer Adcoms or any other amp below $1000.00 used can come close to that kind of electrical pressure.Adcom 555 mk2 is also another good one to look at.Because of the direct coupling theres no capacitors between stages and no power limiting devices. This is good but care should be taken because these amps seem to pick up any change or noise in the ac outlet. A good power conditioner or dedicated outlet will drop the back ground hiss to almost nothing.The back ground in the music gets darker and everything else stands out like it should.Musical concepts offers mods for just about all the Adcoms if this is what you seek.

Good Luck!

Thanks guys.

Didn't think about the 500 series amp. I owned a 555II about 10 years ago.