Is the QED cable that you are using called the QED silver anniversary ?. If so they sound lean in nature. Try changing to thick copper cables first like the Van Damme 6mm squared before changing your components
NAD C370, they sell for £250 - £300 on eBay in very good condition. NAD C370 will have plenty of power and grip. You will still be left with around £400
Arcam SA30 = £2000
REL T5x subs ( BUY 2 ) = £700 each ( Total = £1400
Therefore total = £3400 = $4150
or instead of Arcam SA30 amplifer you can buy DENON PMA2500NE = £2000 and combine with TWO REL T5x subwoofers