Speaker simulators are available across a range of prices, with facilities varying from model to model. Invariably the cheaper ones don't include a dummy speaker load, sometimes known as a power soak, which means you have to keep your speaker connected while working. If you're using a simulator box that can be run directly from your amplifier's preamp output socket, you may be able to use the Master volume control to turn the speakers down or off, but if you want to record the sound that's present at the speaker output, or if you don't have an amplifier with a master volume control, then a dummy load will make your life a lot easier -- and quieter.
Valve amplifiers in particular don't like running with their speakers unplugged -- they are designed to run into the load of a loudspeaker, and without this load, the output stage of the amp could sustain serious damage. Think of it like a car engine; on the motorway you might be able to run it flat out without damaging it, but remove the load by jacking the wheels off the ground and the engine will over-rev and wreck itself.
Fortunately, it is very easy to build a dummy load using resistors that can be plugged into your amplifier instead of a speaker. These are special high power resistors and they'll get quite warm because of the energy they have to absorb, so it helps to bolt them onto a piece of metal plate or a heatsink to dissipate the heat.
The simple circuit uses four 3.9 ohm, high-power resistors wired in series to give a total load of just under 16 ohms. Many amplifiers are designed to run into 8 ohms, but they don't usually object to a 16 ohm load and the higher resistance will mean less dissipated power and consequently less heat. If the amplifer has an impedance selector on the back, leave it set to 8 ohms, unless you think the sound is suffering in some way.
The resistors should be connected using heavy gauge wire -- the copper earth wire in mains cable is fine -- and make sure the soldered joints are sound before you test the finished project. Probably the most elegant way to present the power soak is to mount the resistors on the inside of a die-cast metal box with 6BA or M3 nuts and bolts, using a jack socket to accept a speaker lead. The resistors specified are rated at 25 Watts each so the maximum continuous power this load will handle is 4 x 25 Watts, or 100 Watts. If possible, run at a lower power, as the load could get uncomfortably warm after a time, though the power reduction resulting from using a 16 ohm load will help in this respect.
The resistors are type P3.9 ohms and can be found very cheaply in the current Maplin catalogue. It's worth getting a copy of this catalogue, as many of the bits and pieces so necessary in any studio setup are to be found there, including metal boxes, nuts and bolts, plugs and sockets, cable and so on.
If you have a multimeter, check the completed project to confirm that the resistance at the input socket is around 16 ohms. If so, plug it in and enjoy the peace and quiet!