In the case of your 32.5, the insurance company would probably regard the cheapest pre- in the current Naim range as equivalent, and would settle the claim on that basis. Nowadays this would probably involve putting you in touch with a local dealer with whom it had negotiated a discount. If you insisted on cash, the most they would pay you is what they'd have ended up paying the dealer, and they often negotiate substantial discounts - 30-35% is by no means uncommon. On the plus side, dealers are usually quite flexible with regard to providing alternative products. Your worst case scenario is that you end up owning a new amplifier you don't want, but which will be able to sell for a price far exceeding the cost of a replacement 32.5.
Cables etc. will be replaced similarly, although you may find people distinctly bemused at the fact that anyone could pay that much for a kettle lead or a bit of wire.
The key is proof of ownership, so keeping receipts, and photos of the equipment will help inordinately in settling the claim.
As to specialist insurance, it's important to realise that virtually all insurance policies have a "High-risk items" limit and a single-article limit. High-risk items always includes jewellery, watches, coin collections etc. and commonly includes audio-visual equipment and is set at 25% of the sum insured or a figure like £10,000. The single article limit is commonly £1,000 - £2,000. If you fall foul of either of these, you will not receive the full amount you've claimed for in the event of a claim. If you break the high-risk items limit, there is a (small, but still real) risk that the whole of your claim will be repudiated on the grounds that the company would never have accepted the risk if they'd known about it.
If you have equipment that costs in excess of these figures you will need to declare them as specified articles to your insurer, and pay the additional premium, or buy a policy with no limits from someone like Hiscox.