The previous two posts both make excellent points. Whether the passive sub makes a difference to you really depends on where you see yourself with home cinema.
If this is your end game and you've no plans to upgrade and build a better cinema system then it not really important other than the small performance hit because the amp is powering the sub as well as the main speakers. On the other hand, if this is the first step on the road to bigger and better things then you need to think about the cost of upgrading - that's both in the cost of new gear and what your older stuff might fetch. It's slightly disheartening to think about selling something before you've even bought it, but it should be a consideration to help you way up the pros and cons.
A passive sub works like all the other speakers. It's a speaker in a box. The power to drive that comes from the amp. Bass takes a lot more power than the small satellite speakers. Passives are a method for manufacturers to keep costs down when making entry level gear. An active sub is more expensive to produce because it has its own built-in amplifier. The benefit is in sonics; first because it lightens the load on the main amp by not having to drive the power hungry bass, and second because the sub amp can be better matched to the needs of the speaker it is driving. The bass is stronger as a result.
If you plan to upgrade then selling speakers that include a passive sub on their own is difficult. Who is going to buy them?: They're not an upgrade for anyone. The only people that can buy them are those with an equivalent budget amp that has speaker sockets for a passive sub. If you sell the amp first then you've go the problem of how to power your passive sub. It's chicken and egg. None of this matters if you don't plan to upgrade, but if you do then you need to sell all as a complete system.
On the positive side, the fact that the Yamaha doesn't directly support HD audio decoding directly isn't so much of an issue as it first appears. Any decent Blu-ray player can be set to output Dolby True-HD and DTS Master Audio as a multichannel PCM signal. This means it's the player doing the decoding rather than the amp. To all intents and purposes the resulting sound is identical. The only difference is that you don't get the sexy DolbyHD or DTS-MA logo light up on the amp. The situation might be different if you're using media streamers with ripped BD content.
Where the lack of HD audio might be an issue is at resale time. The typical newbie buyer won't be as familiar with audio formats, so they're likely to skip something without direct HD audio support without realising it's not a big deal.
Is there something better???
The Pioneer and Yamaha are £249 and £269 respectively. Is there something better for the same money..... In short: No. But if you're prepared to tip in just a little extra over your original £300 budget then you can get something much better. Both the Pioneer VSX323 and Yamaha RVX375 are available at £149, though you'd have to join the VIP club to get the Pioneer at that price. Add the Tannoy SFX5.1 kit £169/£149 on VIP and then chuck in a bit of cash for cables and you're in business. No issues with HD audio. No issues with passive subs.
