The deck is just fine to power the speakers. The power of the deck is of no real concern as far as damaging any speaker. Moreover, an after-market deck does enhance the sound produced by your speakers because it supplies more power and if your speakers are stock the sound they produce may distort at lower volumes due to the power supply of the new deck. You'll be fine as long as you keep the volume at a moderate level as to prevent your speakers from distorting constantly. There is nothing to really worry about though. A deck, however, will not do the job an amplifier will do as far as power supply.
If you ever decide to get new speakers or purchase an amp for the speakers just make sure the rms wattage of the amp matches the rms wattage of the speakers it powers. Don't pay attention to max wattage ratings.