I assume you are using the PVR150 to grab TV from an external source. If thats so then your just seeing the quality loss when your PVR150 digitises the video. There maybe some parameters you can adjust to improve things inside Myth but I am not familiar with it enough to help you there.
If on the other hand you are comparing the "off air" picture your TV receives to the same signal delivered via the PVR150 TV encoder then I would say you are seeing a quality possibly more to do with how you are connecting your PVR150 to your TV.
MPEG record specifications:
MPEG2 record datarates:
2MBit/sec, 4MBit/sec, 6Mbit/sec, 8Mbit/sec, 12Mbit/sec.
Selections for DVD Standard play (8MBitsec), DVD Long Play (4MBit/sec) and DVD Extra Long Play (2.5MBit/sec)
NTSC format* at 29.97fps: Full D1: 720x480, MPEG1: 352x240
PAL format* at 25fps: Full D1: 720x576, MPEG1: 352x288
Audio capture formats: 32/44.1/48 KHz, 16bit stereo, 192/224/384Kbits/sec
Video digitizer: 10 bits
Chroma sampling: YUV 4:2:2
Four line adaptive comb filter for both NTSC and PAL to remove dot crawl
Fast video locking for security camera applications
so.. the max resolution is 720x576, but in the real life, the pal resolution is 768x576. other big weakness : Video digitizer: 10 bits. for good quality grabing at least 12bit, but 16bit need.
some 1000 years old bt848 pctv card outpreform this values...