Q: How can I make my drums and my bass guitar sound heavy?
Ah... you're suffering from lightweight drums and bass syndrome! This is very common in recording.
Let's look at the bass guitar because this is more likely to be suffering from this problem.
The principle cause of LBGS (lightweight bass guitar syndrome) is the way the ear perceives sound. When you plug your bass guitar into an amplifier and speaker and turn up the volume, your ear doesn't only perceive loudness, it interprets that loudness as heaviness too.
The microphone doesn't. It picks up the sound exactly as it is - merely loud.
When you set the gain correctly on the preamp, the resulting recording will be hardly any heavier than if you had set the volume control on the amp to 1.
Indeed, it might have no apparent heaviness at all.
So what's the cure?
The solution to this problem is to recognize that loud sound stresses the ear and causes distortion in the hearing process. So in other words, you're not hearing the sound as it actually exists, you are hearing your ears' interpretation of the sound.
Having realized that, you can begin to restore lost heaviness by adding distortion into the process.
This is best done at source. So if you have a powerful, clean bass amp, you need to exchange it for one that creates more distortion. So you need tubes rather than transistors, and low-power rather than high-power, so that when you turn the amp up, it distorts more.
Your speakers too might need attention. Speaker technology has 'improved' since the 1960s and it is possible to design and build drive units that are ultra clean at high sound levels.
But this doesn't produce a heavy sound in a recording. It is better to use drive units of 'old school' design where the cone bends more and produces a more distorted, but heavier, sound.
You might choose not to use an amplifier and speaker at all, and record through an amp simulator, or use an amp modeling plug-in.
Amp modeling is an amazing technique that can mimic the sound of real amps and speakers. The problem can be that although the modeling seems good, the result is lightweight.
Here you can improve the sound by putting the modeled signal through an amp and speaker, which could be your monitor system with appropriate settings of the controls. Mic this from a distance of a meter or more so that you pick up some of the ambience of the room.
You can use this alone, or mix it in with the original modeled signal. If you mix it, consider time aligning the two signals so that you don't get cancelation of some frequencies.
This technique will require a lot of experimentation to get the sound exactly right. But what you will end up with can potentially blend clarity and heaviness in exactly the right proportion.