I don't know the best reccomendation fo ryou, but I do know that media centre PC's (rather than just a hard disk MP3 box) can offer a large degree of flexbility that you may not realise you want.
Ultimately, any old Pc box with a decent sound card (or digital out to your own DAC/surround processor) will work for what you want. But rather than just play MP3's you could have the ability to surf the net in your living room, play divx movies, games, use it as a TIVO box and watch your programs when you want and all sorts.
Quite frankly, with the exception of the tivo part, this can all be done very, very cheaply. You can buy the proper media center PC's but they aren't cheap.. But all you really need is a barebones system and an SB X-fi sound card. You could probably build yourself a decent media centre PC for under 200 quid - the problem will be it sits in a desktop case and may be bulky and give off fan noise unless you invest in especially quiet components (at a mild price premium). Media centre PC's, unless you plan on decoding blu ray and such, do not need tonnes of power. Especially if you only plan on music listening.
A quick go at overclockers.co.uk gives me:
Product Name Qty Price Line Total
Intel Core 2 Duo E2140 "LGA775 Conroe" 1.60GHz (800FSB) - Retail £35.99
Asus Vintage V3-P5945G Barebones System - Intel Core 2 Duo £63.99
Creative Sound Blaster X-Fi Xtreme Audio 7.1 Sound Card - OEM (PCI-Express) (30SB082000000) £22.99
Toshiba SD-H802A 2.4x HD-DVD ROM £61.99
HIS ATI Radeon HD 2400 PRO 1024MB DDR2 HDTV/DVI (PCI-Express) - Retail (H240PRHMF256-R) £29.99
Sub Total : £214.95
Shipping cost assumes delivery to UK Mainland with:
City Link Parcel Next Day (Delivered Mon-Fri)
(This can be changed during checkout) Shipping : £8.95
VAT is being charged at 17.5% VAT : £39.18
Total : £263.08
And that will be able to play you HD DVD's or add an extra 30 quid to make it a blu ray player. You will need an operating system and playback software to go with it though. Umm, doh, add an extra thirty for a 160GB hard disk drive
you can even network this with other computers in the house anyway and stream your movies and MP3's.
One thing you will need though is computer expertise, or a friend/spouse with them.