Spend your money on a good DAC that includes a good clock. Spending thousands on a clock is a huge waste of money, IMHO.
As mentioned earlier, the main purpose of an external clock is to synchronize multiple devices, not to improve sound quality.
If there is an improvement in sound quality, it's going to be much less than you could have realized by putting your money into a great DAC. Lipstick on a pig and all that...