Your small bookshelfs specs:
Frequency response (±3dB): 68Hz - 20 kHz
Nominal impedance: 6Ω
Minimum Impedance: 4Ω
Sensitivity (2.83v@1m): 86dB
Aren't exactly easy load for amp, but :
Unless you unhappy with loudness you get from existing receiver, you should keep the yammi and invest in better sub. I reckon even in UK 200 pounds should get you nice sub. Don't be shy to look in 2nd hand market.
your current under 8" ported sub is most likely severally lacking, especially you have it in large space. This would be much better money investment for you.
If You agree with me, you next step would be to post your room size, music/movies type preferences and which options are available to you at your 200 pounds budget