@Nick Squire it's worth understanding that the
SMSL Q5 PRO and
Fosi HT4S represent two different amplifier topologies -
the Q5 PRO is a class D amplifier with a
digital processing input (TAS5508) - so no additional DAC is required for digital sources - such as optical S/PDIF, which I presume you are using from your TV.
The Fosi HT4S is a conventional
analogue class D amplifier, which includes an
inbuilt DAC for digital sources. It's difficult to obtain technical information on exactly what type of DAC it contains, but I have a suspicion they use a combined Bluetooth receiver/DAC - probably the ATS2853 - since that chip is used in several of Fosi's other products.
If my suspicion is correct, the HT4S's conversion of optical S/PDIF to analogue will be mediocre-to-terrible - and this might explain what you are hearing.
I have 2 suggestions:
i) the equivalent of your SMSL Q5 PRO today is probably the
Aiyima D03 - presently US$130 on AliExpress.
ii) but a better option - get a good quality DAC
plus good quality amplifier - and keep these two functions separate;
Fosi V3 amplifier - presently on sale with 32V power supply for US$70
plus
SMSL PS200 PRO DAC - US$80