You need to prioritize what you want to accomplish with your next purchase. If cost was no object, I'd like to have towers that matched my speakers, unfortunately for most of us, cost is a big concern. The question we all struggle with is how to best incorporate what you can afford into your existing system.

If you are going to use your new speakers in a 7.1 system more than for 2 channel listening, you definitely are better served getting the best possible match for your existing system. I'm not familiar with the speakers you already have, but generally, 2 more that match the current speakers is the best way to go. A good speaker is good for home theater and music.

Tower speakers have their advantages. If your sub is outmatched by the speakers (up to you) then you may be better off with towers and shutting the sub off altogether for music. In a system with no subwoofer (or a poor sounding sub), the added bass is a real bonus.

In a system with a capable subwoofer, I'd always recommend going with the smaller models at a given price point. By design, smaller speakers with fewer drivers offer many sound advantages. Introducing more woofers and larger cabinets is extremely hard to do without incorporating compromises to the sound. Simple is usually better. For a given price, say, $2000, you'll find a bookshelf model will have higher quality drivers, crossover components, and cabinetry. Higher quality drivers are often easier to work with and should sound better, which can lead to superior design topology over all. You'll generally find that a smaller speaker offer a higher level of sound quality (unless they're unfairly priced) at the expense of some bass extension to a tower speaker.

This is where the subwoofer comes in. A proper sub setup will pick up where the speakers trail off. If the sub can perform at the same level as the speakers in terms of sound quality, you've got the best of both worlds. If you really want the best sound for music without detracting from your home theater experience and your existing sub is up to task, this is the way I'd go. It's a bit more work to setup the sub, and a lot of subs aren't up to the task, but when everything lines up, it's just a better overall integration in my experience. There's a subjective component here, if you're a real bass head, you might prefer to sacrifice some midrange and high frequency refinement for the added bass.

How to use the sub is an entirely different thread. After years of doing what everyone told me and setting my crossover as low as possible, I've finally realized that a slightly higher crossover is preferable for a number of reasons I won't get into right now. Actually, I think it is setting the XO as low as possible, but most people overestimate how low they should be setting their XO's optimally. Room treatment and eq-ing can come into play. It can be quite a task to set the sub up properly. Once you get it though, there's no comparison, IMO. Sometimes it's just easier to go with the towers, which is certainly a valid reason. In my 2 channel rig, I have tower speakers and a sub. The XO is a bit lower than my 7.1 system (all bookshelf speakers), but still higher than the -3dB ability of the tower speakers.

As you can see, there's plenty to consider and I doubt you'll find a universal answer to your question, but hopefully you carefully consider all of your options and pick the solution best for your situation.