Slam or attack is in the upper midrange 1 to 3 Khz (when a drum stick hits the skin or a piano hammer hits the strings). If you cut this upper midrange then it will sound laid back (many speakers do so and sometimes deliberately so). Another effect is bass. Good transient reponse from critically or over-damped speakers will mean that transients do not last any longer than they should (sealed subs and speakers with no reflex ports are often the "tightest" and cleanest sounding). Ported designs with amazing bass for a small box are often culprits that give a lazy or "one note" bass sound. Box resonances and woofer resonances (ringing) will also muddy or slow the sound by adding distortion (boosts especially in the 500 to 800 Hz range will add warmth).
Note that many speakers (especially two ways) use a 6 or 7 inch woofer to cover bass and midrange - inevitably many of these designs beam above 1 Khz (the woofer is just too big) and to varying degress give a laid back sound. Beaming means they excite just the sweetspot with a flat response whilst the rest of the room reflected off axis response is heavily down until the tweeter kicks in between 3 and 5 Khz adding that unatural "hi-fi" sound where the lead acoustic guitar may be just a wee bit too clear to be convincingly natural - kind of like there is a microscope on it...again some speakers are deliberately designed this way (remember as much as half of what you hear is reflected)
Another factor is dynamic response - this is very counter intuitive. If you compress the sound and it lacks dynamics then the sound actually becomes perceptively louder or edgy or snappy. This is because you have raised the root mean square energy (energy over time). Teh sound can be very tiring and fatiguing and is common on pop CD's today but especially in Punk and Heavty metal such as Metallica or Green Day. A highly dynamic system with a highly dynamic recording (contrary to what you would expect) will sound softer, more natural and relaxed, effortless and less fatiguing even at higher SPL.
