The NAD 71xx series were very good products in their day. The amplifiers had 5 db of dynamic headroom, something that was unusual for solid state equipment then and now, so a 20 watt receiver (using FTC RMS specifications) could provide real usable dynamic peak music power of about 50 watts. These receivers also had very good phono preamps. I don't know about the headphone circuitry, but maybe someone else can comment here.
The 7125 is highly regarded for its sound as well as for its relatively small size, something the more powerful 7155 cannot equal. With the exception of possibly the Advent 300 receiver, another vintage receiver in great demand, there are few if any really small footprint receivers that can be considered to have a great sounding preamp/phono stage, tuner, and amplifier in a such a small package. For example, the Marantz 22xx series, with the exception of the smallish 2216, are all built on a much larger chassis. This has certain advantages, such as better airflow with cool running circuits, and very sensitive and quiet tuners, powerful amplifiers, and quiet preamp/phono stages with a very capable headphone amplifier circuit. However, these units are all much larger than the NAD 7125, and also somewhat larger than the 7155, but here the difference in size is not so great.
NAD equipment was built to be able to sell for a competitive price, as a result some design shortcomings were present: such as the power switch and other push buttons directly mounted on the PC board. If the button is used long enough, the mounting on the PC board will weaken and the board can crack or develop breaks in the trace wiring. No spare parts are available except to buy another used unit if you can find one. Also, at least for the 7125, the tuners on these were first generation digital type tuners, and tend to flake out or die with age.
A working 7125 in good condition for about $100 would be a great find. A working 7155 could be a good deal, but it should not cost much more than the 7125, perhaps up to $150 or even less.
At least with the vintage Marantz gear, if the power switch dies (something that is known to many times happen), at least the switch is replaceable with an aftermarket part, or a working switch salvaged from a parts unit.
