A valve amplifier against a solid-state one. They are not really rivals: they differ in character, in what they will drive and in what owning them involves, so start there rather than with the watts.
| Area | Better | Why |
|---|
| Power | B | McIntosh MSA5500 has more power - 100W against 70W into 8 ohms. That is about 1.5dB, which is a small step in loudness but real headroom on peaks. |
| Built-in DAC | B | McIntosh MSA5500 has a DAC on board; the other needs an external one, which is another box, another power lead and another purchase. |
| Phono stage | depends | McIntosh MSA5500 has a built-in phono stage and Audio Research I/70 does not. Decisive if you own a turntable, completely irrelevant if you do not - and an outboard phono stage is easy to add later either way. |
| Use with a TV | depends | Only McIntosh MSA5500 has HDMI, so the television can drive it directly and one remote controls the volume. If the system is for music only, this changes nothing. |
| Room to grow | evenly matched | The same expansion options on both. |
| Amplifier type | depends | Audio Research I/70 is Tube and McIntosh MSA5500 is Solid State. Valve designs are usually warmer and softer at the limits and need their tubes replaced eventually; solid state is the lower-maintenance choice. This is a taste and ownership decision, not a quality one. |
| Shelf space | depends | Audio Research I/70 takes up about 43% less shelf area (1428cm² against 2512cm²), and it is 225mm shallower, which is the measurement that decides whether a rack door shuts. |
| Price | B | McIntosh MSA5500 costs less - $8,000 against $9,950 at list. |
| Watts per dollar | B | McIntosh MSA5500 is the cheaper watt - about $80 per watt against $142. A blunt measure, and it says nothing about how either sounds, but it is the honest one for raw output. |
A Audio Research I/70 B McIntosh MSA5500
depends neither is better in the abstract - it turns on your room, your system or how you will use them. Only specs published for both amplifiers are judged; anything missing on either side is left out rather than guessed.
Which should you choose?
McIntosh MSA5500 is the easier recommendation: it leads on most of the measured areas and costs less. Take Audio Research I/70 if one of the "depends" rows above describes your system. If you play records, McIntosh MSA5500 saves you buying a phono stage; if you do not, ignore that row entirely. If this sits under a television, McIntosh MSA5500 is the one to buy - HDMI ARC means the TV remote controls the volume and there is no second box in the way.
What a spec sheet cannot show: how either sounds with your speakers, how the controls feel day to day, and what the app is like to live with - which is where most all-in-one amplifiers are actually won or lost. Power figures are the manufacturers' own and are not measured to a common standard, so treat small differences as noise. If you can hear both, that will tell you more than any row above.