I think Nick Peacock is right. We are comparing two very straight forward stereo amplifiers.
Though I don't know much about the internal workings of a digital amp, I know that, much like switching power supplies, digital amps are very efficient. That is good for the manufacturer, but not necessarily good for the consumer. It means smaller heat sinks and smaller power supplies for the manufacturing process. But, for the consumer the results are variable. Some digital amps sound fine, others, not so much.
I think in at digital amp, but remember my knowledge is limited, the analog signal is changed into a pulse train, where the pulse width and amplitude reflect the original signal. That pulse train is then, later on converted back to analog on the output.
This is the same principle in switching power supplies. If you ever lifted any old electronic equipment, you know they weigh roughly the equivalent of a boat anchor. That is because of the linear (think analog) power supplies. Switching or digital pulse power supplies can use much smaller and lighter transformers to yeild the same power as an linear power supply.
Lower cost, lower heat, but equal power. Very efficient.
I suspect the same principle has been applied to digital amps. They have found a way to make the power output of the amp variable in proportion to the input signal, but to use less power along the way.
This description is more of an illustration, than an example, but I think it servers the purpose.
Now there are some hardcore Old-School people out there who simple would not have a digital amplifier, but I say, give a listen and if you like what you hear, then problem solved. I'm personally uneasy about them, but if I heard one I liked, I wouldn't have a problem with it.
I think Onkyo has a problem with the Continuous FTC/RMS power rating. It is indeed the most difficult. In Europe, Onkyo is not real forth coming with their power ratings. Rather than continuous, the use DIN and EIA, as well as Dynamic Power ratings. Those number tend to be very high because they are easier power tests. But, they also reflect more real world listening situations, where as the more standard FTC/RMS does not reflect actual listening.
So, you need to read the power specs very carefully on the Onkyo's.
For example, the Onlyo A-9377 is 90w to 4 ohms (EIA), and 80w to 8 ohms Dynamic. Which implies about 40w to 50w by an estimated continuous standard to 8 ohms. That is still decent power.
However the Digital A-933 is 80w to 8 ohms (DIN), and 110w to 8 ohms Dynamic Power. That makes this a more powerful amp than the A-9377. Not hugely more powerful, but definitely more. I'm guessing something in the 50w to 60w range, though I can't be sure. Remember, Dynamic Power best reflects actual music listening.
Keep in mind, I'm not criticizing Onkyo amps. I've got an older TX-8011 Receiver which is about 50w channel. I'm very happy with it, and got it for a very fair price.
So, in general, the Digital A-933 is a more powerful amp, and cost roughly £400 to £500. The Analog A-9377 is a less powerful amp and cost roughly £300.
It you need the power, and like the sound, and can afford the extra money, there is nothing wrong with the Digital amp. In terms of features, the A-933 doesn't appear to have tone controls, but a small panel flips down to reveal tone and speaker switch controls.
Again, I would ignore the Digital aspect, and concentrate on cost, sound, and features. But also look at other amps you can get for the same price.
NAD, Cambridge Audio, and Marantz are good alternatives in that price range. Also, at closer to £600, the Rotel RA-06SE. Near £500, you should be able to get more powerful amps in the roughly 70w continuous to 100w continuous to 8 ohms.
At closer to £300, the 50w/ch continuous to 8 ohms NAD C326 is extremely highly rated.
If you are by chance looking for a high value amp, meaning a lot of amp for a very modest amount of money, consider Harman Kardon. Either the HK3390 Receiver or the HK980 integrated amp, both with 80w continuous per channel to 8 ohms, are in the roughly £270 to £315 range.
Hope that helps.
Steve/bluewizard