Changing the pre-amp tube currently in your HT-5 will do nothing to change the amount of gain. The 12AX7 in it already has the most highest gain factor (100), in the 12A_7 series. Different pre-amp tubes have different amounts of gain. A comparison of the gain factor numbers can be found
here. A Tungsol is known to have smooth gain, which makes it ideal to be used in the V1 of many pre-amps.
Note: V1 is the designation assigned to the pre-amp tube in the pre-amp section that handles the gain.
However, changing a pre-amp tube can affect the amount of gain, assuming that the tube assigned to the gain stages of the pre-amp section, but of course it must have has a lower gain factor than a 12AX7, like a 12AT7 or 5751. By the same token, some people like changing the V1 12AX7 in the JVM to a 12AT7 to get less gain out of the JVM, but that screws around with the selectable gain stages in that amp. The JVM uses two 12AX7s to get the large amounts of gain that it has.
If you don't know how to change tubes or are not ready to do the swapping on your own or learn how to do so, don't buy a tube amp. Stick with solid state ones. Tube swaps are the most basic sort of maintainence that you need to able perform when owning one, just like changing of strings.