It's a question that's intrigued me for a while now. Reading around the topic, the experts seem to be saying that broadly, correctly designed amplifiers should all do the same job and should not significantly affect the sound.I've recently installed 3x Kef LS50 speakers as front left, right and centre in my home theatre and love them to bits. They are supplemented by a pair of SVS PB-1000 sub-woofers and the whole thing sounds great powered by my a/v receiver, a Marantz SR7010.However, I couldn't help wondering if there was more to be had in terms of volume, dynamics and transparency, especially for 2 channel audio.The stars aligned and over the Christmas period I managed to get a couple of stereo power amps to play with, a Parasound Halo A23 and a Behringer A500. My plan was to conduct a comprehensive shootout between the two with my SR7010 also in the mix.The two power amps are broadly similar. The Behringer is rated at 125W per channel into 8ohms and can be bridged mono to provide 375W. The Parasound is also rated at 125W per channel and can also be bridged, providing 400 mono watts. Both support balanced and unbalanced inputs.The Parasound is a more refined hi-fi amplifier with niceties like 12v control and a sleek visual presence. The Behringer is much more industrial and is targeted at studio installations.The A23 retails for £1299. You can pick up the Behringer for £180.Wait, what? That was my reaction too. You can't possibly compare those two, right? However, there's been lots of posts on the interweb about the Behringer and how it's a great value giant killer. I had to find out...The plan was to use my A/V receiver as a pre-amp. In Pure Direct mode, it's reputedly an adequate performer and would give me a standard front end to the three amp stages on test. Sources were mixed but I mostly listened to a Pioneer 565 for CD playback and a Squeezebox for digital files stored locally. I would listen in stereo mode only with room correction disabled.One of the issues I had with this setup was that there are no balanced pre-outs on the SR7010 so all testing would be done with unbalanced. No big deal, I thought, as the components were located proximate to each other. and cable runs were very short (< 1 M).However, the first issue I encountered was that the unbalanced phono input on the Behringer was really noisy. Playing any content, I could clearly hear noise/distortion/breakup. It seemed that the noise floor had magnified and there was a bed of white noise with even some breakup in the vocals.I satisfied myself that this was not caused by any upstream components by quickly swapping over to the A23 which sounded perfectly clean using the phono inputs.I'd read that there were issues with the A500 input level controls and they should be set to max, which they were. I also recalled commentators mentioning that the balanced inputs were much better than the unbalanced on the unit.Fortunately, I had a set of phono->xlr adapters to hand and so I tried routing the unbalanced preamp output to the Behringer balanced input. This solved the problem and the music regained the expected clarity. Not a great start for the Behringer. All further listening was conducted with phono outs from the pre-amp connected to XLR ins on each amplifier via the adapters.Time to get started for real. From the first track my Squeezebox threw at me (Apron Strings, Everything But The Girl), the Parasound sounded amazing. There's a slight but perceptible bump in smoothness and openness compared to the Marantz. There's a definite improvement in depth - the image reaches further forward and backward from the speakers than with the Marantz. Nice.My well-laid plans to conduct extensive tests with all kinds of material were quickly confounded. It turned out that the Behringer fell significantly short of both the Marantz and Parasound. Music played through it sounded flat and uninspiring. Compared to either of the other two, the soundstage seemed to collapse completely. Whereas the Parasound provided width and, crucially, depth to music, the Behringer simply fed sounds to the two speakers.In some cases, the Parasound seemed to be truly holographic with sparkling high frequencies such as cymbals, piano and triangles seeming to come from behind the listening position. The Behringer never seemed to be able to generate anything beyond the outer edges of the speakers themselves.I so wanted to love the Behringer to justify getting 3 to power l/c/r speakers but in further listening sessions, the Parasound consistently put a smile on my face with every track I listened to. I honestly hadn’t expected to perceive a difference but there’s certainly an expansive sound to the A23 that just isn’t there with the other two.As a final test, I sat my 13 year old son down and played him a single track on all 3 amps. It was music he was unfamiliar with and I asked him simply to tell me which sounded best and why. He had no notion about what an amp is, let alone brands or value. He picked the Parasound as the far and away winner. He said he could hear the different instruments better and the sound was a lot fuller. The Marantz was second. He said it was very good but not as good as the Parasound. The Behringer he thought was the poorest of the three. Not as clear as the others.Unscientific, I know, but interesting nonetheless.It was a true disappointment given all the good things I'd read about the Behringer. Proponents will argue that it's a studio amplifier and thereby designed to be hyper-analytical, presenting the material warts and all for critical production listening. Maybe so, but it just doesn't sound good in my set up.It's a closer run thing between the Marantz and the Parasound but the A23 does win out for that holographic presentation. The Behringer was sent back and the Halo is now firmly ensconced in my setup, waiting for a decent stereo pre-amp to join it. The Halo P5 looks nice...(Readers may be interested in the discussion thread at avforums that evolved during this test).