My opinion, is that it's an effectively 35 watt amp. I'm sure it's a fine amp within its limits, which aren't very far.
I dunno about the Onkyo, but at the very least you're likely overpaying. The Marantz can most likely deliver a usable ~80w and the Harman Kardon 3490 that gene measured
harman/kardon HK 3490 Stereo Receiver Review — Reviews and News from Audioholics
can delvier a usable ~100w.
The Marantz also has bass management, in case your room is problematic in the bass you leave yourself the option to roll in two or three subwoofers in order to get you tight, accurate, musical bass.
Beyond that, the rega is an integrated amplifier with preamp and amp sections built in. Now this would be fine, except that the rega does not have preamplifier outputs. That means that in case 35 usable watts is not enough for your choice of speakers, room, music, or listening habits, then you will need to replace the entire unit. If it had preamplifier outputs, like the Marantz, you could simply "Add on" an external amplifier like the crown I mentioned.
Now the SR5006 does not have a phono input stage. Do you need one? I can see where that may be a deal breaker but you can always add an external phono stage.
In general, it's something like this:
1)The Speakers, and their interaction with the room, represent almost 95% of what you hear, so long as the electronics are simply decent and within their limitations.
2)Speakers are a reproduction tool to let us hear a recording. We don't listen to speakers to "listen to speakers". Therefore speakers need to be accurate transducers. In order to be accurate, speakers must have flat response in the forward axis, as well as a balanced and smooth response radiated into other directions so that reflected sound does not serve as a source of coloration. They must be free from resonances and must be able to reproduce dynamic transients without compression.
3) Getting truly accurate speakers, is more difficult than getting very accurate electronics. There's no discrete number that tells you which is which, but simply put, a vast majority of speaker companies are not in the business of selling you good sound quality (accuracy) but rather a preferred sound signature (colorations). It's gotten to a point where many electronics companies even choose to follow suit, as difficult as that may be. Never trust anyone's subjective experiences with loudspeakers, electronics, or even silly irrelevant things like wiring, because much of the industry is simply put - confused.
4) Many hi-fi shops are in the business of electronics, with speakers simply being a means to an end. They will push electronics on you, using buzz words like "Class A" and "Microdynamics" to make you spend on electronics, even if the speakers simply aren't very good - and in the process form a bias in your brain against other valid options, some of which have serious merits. Once people buy speakers, they become resistant to changing them, yet are open to the idea of changing electronics. So long as the electronics are what people are open to changing, people never get improvements in their sound quality, unless their previous electronics' limitations or design flaws were sufficiently being exposed.
