I had the Niles SI-275, and it was anything but weak. I had one and I severely regret selling it. It has a robust transformer (approx. 1000kva) 4 - 10000uF capacitors and a total of 12 transistors, that's 6 per channel. I think you'll find the Niles SI-275 will best your old Onkyo in terms of headroom. If there's any possibility that the Onkyo fried due to handling overly complex loads that is a worry that is far far away with the Niles SI-275. That beastly amplifier is rated to safely handle below 3 ohms!
I've got a Sony TA-N611 I've been using and it's rated 150 watts per channel @ 8 ohms. It has 8 transistors of similar value, two 15000uF capacitors, and a smaller transformer as well (figure half or less). It is bigger still than the Onkyo you had been using, which I believe has 4 output transistors total, two caps of 10000uF or less (yeesh) and a small transformer in the size class of a typical 350-400kva transformer.
SI-275 has a current draw of up to 9 amps, the Onkyo is rated at 2.5 amps. The SI-275 doesn't weight a lot more, but it has a smaller case that doesn't waste space like the Onkyo. Instead of having a heavier less efficient iron core transformer like the Onkyo the Niles has a nice toroidal.
The auto signal detection is very nice because it doesn't require the use of a 12v trigger, or you getting up to turn the amplifier on. The amplifier has good features, solid power, excellent headroom, and it's low profile and efficient (it runs extremely cool and this means its power is being put were it should be, into your speakers).
Street price is $400-600 on the Niles SI-275.
I looked into the SI-2125. I'm not sold on that unit. It seems to have a completely different design approach. That's not to say there's anything wrong with it. I'm just not certain that it's that special. It's current draw is 8 amps apposed to the current draw of the SI-275's 9 amps. The SI-275 weighs 21 pounds and the SI-2125 weighs 26 pounds. The SI-2125 has a larger chassis, possibly accounting for the increased weight.