Yes, you can use an amplifier more powerful than the attenuator/rated power of a Torpedo product. With a Torpedo Studio or Reload, you can use an amplifier rated for more than 150W. With a Torpedo Live, Captor X or Captor, you can use an amplifier rated for more than 100W. But please remember “you just have to keep the output volume of the amp low enough so not to damage the attenuator”.
The Torpedo products with a build in attenuator are rated for a specific power (measured in Watts). This means that the internal load box can dissipate up to this amount of power (assuming proper ventilation). A Torpedo Captor X for example can dissipate anything between 0 and 100 Watts.
If more power is fed into the Torpedo, it will overheat, and go into overheating protection. You will be warned about this, by means of a visual indication, a volume drop, and/or a mute of the sound, depending on the Torpedo product. So, in order to prevent the Torpedo to go into overheating protection, you have to fed it no more than its rated power.
How to do this safely
Every amplifier is rated for a certain amount of Watts. This means that the amplifier is capable of outputting “this” much power. However, this does not mean that the amplifier actually outputs “this” much power in any situation: a 150 Watts amplifier does not output 150 Watts as soon as it is turned on. The output power is related to the actual output volume of the amplifier, which is dependant on many things (amount of distortion, EQ, output level of the guitar…), but is mostly controlled by the Volume or Master Volume control. So if you keep the output volume of any amplifier low enough, no matter how powerful it is, you can make it output less than any chosen amount of power.
Of course, it is almost impossible to know what the actual output power of your amplifier is in any particular situation. In particular, please note that the power output is in no way proportional to the Volume control: when the Volume is set to 50% (12 o’clockish), the output power is never half the rated power of the amplifier! It can be 10%, 90%, or even already the maximum power. So, if your amplifier is rated for more than what your Torpedo is rated for, you still can use it, you just have to play it low enough. For this, you will have to use your best judgement.
Some tube amps, mostly vintage (or from vintage inspiration) ones, can output more power than advertised. This is because power rating used to, and sometimes still is, given when the amp is played approximately clean. But when played distorted, the amp can output more power. In this case, the actual power output can usually go up to 1.5x the rated power. For example, it is not uncommon to see a 100W vintage non-master volume and/or single channel tube amp outputting up to 150W when cranked up to max. As a general rule, if the amp has 4 output tubes (typically 4 6L6 or 4 EL34), it can probably output more than 100W when played distorted at high volume. You should keep that in mind when playing them on a 100W load box/attenuator, like the Torpedo Live, Captor X or Captor.
A 120W amplifier is not that much more powerful than a 100W amplifier. These 20 extra Watts are likely to be there only if you play the amp loud. At moderate settings, it will probably output less than 100W. However, the point just above still applies. In short, you are probably safe using a 120W amp with any Torpedo product/attenuator rated for 100W or more, but keep these limitations in mind and proceed with caution.
Very powerful amps can still be used with Torpedo products, but care should be taken when using them. Here at our offices, we’ve been using a 500W bass amplifier for years on all our Torpedo products with an attenuator without any problems, so it’s not impossible. We just, as already stated, keep its level way down.
Generally speaking, a good starting point is to keep all the settings of your amp just like when you are using it on an actual speaker cabinet.
And if you have any doubt, don’t hesitate to ask the manufacturer of your amp about its actual output power in your particular situation.
When using your amplifier with a Torpedo product for the first time, monitor both the amplifier and the Torpedo. Does the amplifier sound OK? Do the output tubes of your amplifier heat more than usual? Do they turn red? Is the air flowing out of the Torpedo really hot? If anything seems wrong, lower the volume of your amplifier. If everything seems OK, you can go on.
On an adjacent topic, please refer to the user manual of the various Torpedo products for notes on what the ideal output volume is for your amplifier (regardless of its rated power). Playing an amplifier as loud as possible is not always the best idea.